Just throwing out thoughts - and with Crintera, they are the remaining Domus Magnus.
So.. just th list given above, for now?
That are commonly known about. There could be others, but we are simply too junior to know about them.
Which, while not a Mercere Portal, certainly seem like a form of portal - especially since it appears to circumvent the Aegis (or punch right through it).
https://forum.atlas-games.com/t/spell-effect-to-allow-casting-underwater/7390/19
Probably need to continue this here.
I don't object to the spell, per se. I object from two people being able to learn Hermetic Arts from the same source at the same time. At the very, very least, it needs to subtract from the SQ, much like a teacher does when multiple people are taught by a teacher, except a teacher cannot teach multiple people Hermetic Arts.
If the spell/device is going to be in use here, I think it should be flavor, as in someone is studying from the book, without anyone else knowing what he's studying. Things then fall apart when someone tries reading that book physically while another is reading it via the spell/device.
No. I wrote that, to explain why I think you're being silly. 
(And I mean that in the nicest way possible, not as an insult!)
Feel free.
Yes, I've had a lot of those problems. Back in the third edition when you needed every book on a given subject to study that subject.
In 5th edition, with tractatuus, that has largely become irrelevant, because there has usually been another book on the subject.
Ofcourse, it may be that I have simply played mostly in covenants of unusually prolific writers, but that has always seemed the most sensible covenant service.
I do not disagree with the idea of one-on-one, but it's too easy to get around.
But can't be bothered to hire a scribe?
Yes, a grand total of about 1 mythic pound per year, if I recall covenants correctly. It might be 4 pounds per year. Still not a major issue.
That was not your orignal argument.
It also does not mean that there is anything mechanically wrong with the spell, which was what (I thought) was being discussed in the thread.
Now, I not that you never got back to the subject of scribes, so I will.
So far, I have invested 14 points in a mirror with the Endless Repetition of the Bottomless Pool, specifically to limit debates over library resources.
I did not know they were considered desireable.
For the same 14 points, I could hire 2 highly competent scribes, or 3 less competent ones, which would probably be more effective, as scribe score is irrelevant for copying tractatuus. Technically, I could get them for free if I could be bothered to stat them out - which is still not off the table.
2 (or worse 3) scribes can copy the library faster than we can read it.
Titus has plenty of Arts with scores of 0 - he'd happily study some of them for a few seasons while whomever was hogging the good books got bored with them.
That much faster to the apprentice-taking competence, yes?
He could even wait that extra half-a-year before the scribes had copied whatever books he really wanted to read.
He would them be able to read these books at his leasure and worse, We'd still have the extra copies of each and every book he'd bothered to have copied!
This would mean that not just Titus would have an easy time of finding access to books, but as soon as the scribes had done their jobs, everyone would.
Furthermore, these copies are open for trade, or if we do not own them, the copy could simply be sent to the original author (or his/her heirs) with a note stating that we'd enjoyed the book so much that we wanted to help them spread it even further.
Note that the Cow and Calf does not prohibit you from making copies, it simply states that the right to distribute copies belongs to the original author. An Lo! We've even proof-read the copy.
For these reasons, I could see (and indeed have seen) troupes have trouble with scribes, especially in large groups, but not with the Endless Repetitions.
It even demands time from the magus (and a pawn of Vim vis) whenever the collection has grown such that the catalog (Covenants, p. 98) needs updating.
Now:
This is a legitimate argument against having hordes of scribes. Almost.
Covenants p. 71 indicates that resources to write (or copy) 4 books cost 1 mythic pound. Which I trust you will find almost irrelevant, compared to eg. the upkeep of buildings or provisions for our horde of minions.
EDIT:
Editted because I'd forgotten to write a bt, sorries.
Now, in this thread I posted the following, in the (appearantly false) assumption that if there were any complaints or comment, such would be noted in that context.
Merely a cutesy, really. A perfectly alid argument can be made that we are (supposed to be) a medium-power covenant, which limits us to effects of level 40 or less. This is also the device I was expecting complaints about.
Hard to really argue against, with a direct page reference I suppose.
I'd have prefered a non-skeletal version, magically preserved. But them's the breaks.
This one suddently turns into a potentially major issue, because there was an unstated desire for a specific type of stories that I had not seem as having priority. Funnily enough, this device was intended to be a non-controversial convenience, specifically designed without penetration, so it couldn't be used for disreputable stuff like oh... reading books from another covenant's library through their Aegis.
Ok. No penetration. Does the book then come within a magis's Parma when he is touching it?
I don't have a problem with the device existing. I have a bit of a problem with it being used to bypass resource congestion.
I wasn't expecting it to - books are usually placed on a stand/lectern while you read it (Covenants, p. 121, box, Bookstand of Hespera for an enchanted example). If you really need it to fall under Parma , I suppose I can live with that.
It was pretty much aquired to circumvent resource congestion, because I didn't see it as desireable.
So... how big a problem?
PS: As far as I am concerned, we could all be using Endless Repetition, but that would be rather expensive, as epople would need either the ability to cast a level 35 spell with a requisite, or purchase each a fairly expensive device.
PPS: Would it be better if I sacrificed a bit of power on the Key to Safety (or redesigned it entirely/spend the points differently) and had each of the Mirror of Endless Repetition and the Tireless Servant (possibly somewhat re-designed) keyed to Titus so that he was the only person able to use them?
He'd still by-pass some of the congestion, but atleast he couldn't lend it to others?
But resource congestion is desireable. It generates stories. I offers opportunities for players to make trades and discuss their order of study. Further, it forces character into hard choices. For example, in Canaries are Dying, I've had to postpone studying from a Magic Theory text three times now. I dislike the idea of any character anywhere always being able to perfectly advance in the order that they prefer, without any cost. IT is the central problem to a covenant, especially a spring covenant. It changes the character of the game, drastically.
Again, I don't have a problem with it existing, but I have a problem with two people learning from the same book at the same time, especially if it is on Arts, as it becomes a better method for teaching than can be done, since you can effectively have many on one instruction. That is a huge problem, and one wonders why schools haven't sprung up if the device exists and works. So, I prefer that a book is only usable by one person at a time, regardless of the method.
I don't find the spell particularly unbalancing. Yes, it can be used so that two magi can use the same book at a time. Not a big deal to me, since from what I've seen it is mostly a matter of rearranging your planning so that you study A before B, or the other way around. Most magi of our covenant will have different interests. Only books of interest to all the magi may have that problem -- for example that big summa on Magic Theory. If it becomes a problem, we would make a second copy of the book. Any decent scribe with a score in MT will be able to copy it in a few seasons*.
Note that even without this spell, magi would be quite paranoid about having any arcane connection to their books lying around, because is an AC is stolen, so can the book itself. The spell just makes it worse, because one can copy the book without the owner knowing. They would be against taking any such connection outside of the covenant. They would watch any visiting magi to whom they grant acces to their library like hawks.
Yes, having such an enchanted item removes some annoyances. But then, that's the point of having resources, isn't it? But it's not a big issue for me, one way or another. I've built up such a long list of things my magus have to study that not having access to a specific book for a year or two won't make a difference.
*I think that the number of points to copy an Ability summa should be level x 5, as it is when it gets written. So our big L8 MT summa would take 4 seasons for a professional scribe to copy, instead of a single one if we just go with the level.
So Arthur makes an interesting observation, and given the structure of the saga, most of the problems are likely to be of the players own design, unless they want to hand something off to another person to SG it for a more random. So in many ways, this isn't really an issue for this saga.
But seriously, why hasn't some enterprising magus started a school with 5 or 6 of these devices? 
Very. Essentially the same observation as I've been trying to explain (in rather more detail) for almost a page by now, but certainly an interesting observation.
Clearly he explains it better than I have. Thank you.
Because of the initial investment?
Because who would trust others with ACs to ther books?
For the same reason of artificial retention of medieval flair that all other things in Ars Magica survive on?
When you think about it, Hermetic Magic can change the setting, irrevocably. This has been extensively discussed.
I can clue you in on some of it if you'd like, but appearantly I'm not very good at explaining things.
Agreed.
To expand on the second one, would you thrust apprentices with AC to your books, if you don't thrust them with the book itself?
There is also the mindset that knowledge is power, and should be earned through hard work. No magus worth his salt would simply hand the keys to his library to someone else.
We tend to forget that, as players, because we live in a society where knowledge is often freely available (if you can be bothered to look for it). And even that is a very recent phenomena.
I never though you were making that observation, to be honest. So, if this is a device that isn't going to be used...why bring it/have it?
No, I wouldn't trust apprentices with it, but I would trust someone with it, who uses the AC and starts the devices, which maintain concentration, and can be manipulated by the students. Someone "in-charge" maintains posession of the AC. Be it a grog/companion/magus level character, just someone who is trusted with access.
The initial investment isn't all that great, to be honest.
I like the device, with the exception of allowing two (or more) people to learn from one book at the same time, it is a completely unreasonable premise to me. There should be no method around allowing someone to learn Hermetic Arts other than one-on-one, whether it is reader and book or teacher and student. If that's a desire, I suggest that it becomes an avenue for research. Maybe after the initial reseach goals have been realized. Allowing it to read from a book at another covenant, whose Aegis you've been invited into, but they don't want to lend the book, it's a bit different. Certainly Durenmar wouldn't allow any of these near any of their books. 
The warping inflicted on the books is an interesting thing to study.
I think you misjudge what Tellus is saying.
There is no hard reason why we should limit this. The spell/device is useful to avoid the relatively small inconvenience of waiting for the book to be available if someone else is useing it. There are plenty of magic that is built specifically to solve inconveniences like that. If you want a way to limit it, personally I would go with: only 1 such effect can affect a book at one time, otherwise the magics interfere with each other. So it would allow 2 people to study from the same book, but no more. Just like a copy would, in short. Doesn't break the setting nor the system.
There is a price for using this effect: Warping. If someone uses the spell as written, he causes warping on the book. That's 1 point per casting, so 80 point in a season for the Sun version for a Warping score of 5 after a single season! That would quickly become unacceptable to magi. An enchanted item (or a D:Month variant of the spell) would cause less warping, but needs time/vis to invent/enchant. And even the lesser warping would probably be deemed unacceptable for the most valuable books. Magi would know that, so this would be used only for the less valuable books. The first time we notice a book being Warped, we'll ask question and possibly ban the use of the effect on some (or even all) of our books. Again, this doesn't break the setting, nor the system.
I'm not sure sure warping affects the book.
Much like I don't believe the recipient of The Incantation of Lighting or Ball of Abysmal Flame doesn't inflict warping on the recipient, I don't think this really should inflict warping on the book. It's the Target, target argument. If it warps the book, it should probably warp the mirrors. If the magic can be interfered with by another person attempting to reproduce the same effect, why not when someone actually uses the book?
I'm basically done, I'm following my own rule, and am considering myself over-ruled, unless Archimedes comes in on my side. If he stays mum, it's fine. I'm not convinced, but I'm clearly over-ruled pending Archimedes feelings on the matter.
Just figure out exactly how it works, what the limits are (and there should be some, perhaps as you described, perhaps including warping, etc).
He argues that resource congestion is unlikely to be a problem - there is a single book he thinks might be a problem, and we could just copy that.
I hope I made that point as well, though it was undoubtably burried deep in that horrible wall of text.
When I picked this mirror, it served several purposes:
A) I like the spell, but do not use it often (because I SG much more than I play).
B) It filled just the right amount of points
C) With this device, I could simply sidestep any elements of resource congestion, which has never been much of an issue in games I've played/run in the 5th edition anyway. An example of a Bonisagian "must solve it with magic" overkill solution.
Alternatively, I can re-spend my private points once again.
No promises on when that can be done - it was hard enough the first time.
I keep feeling you're seeing an issue where there is none, and insist that it is a very important principle.
I find the reference to teaching most curious - I've seen teaching used more in the 5th edition than in any earlier edition, despite the existence of this spell, and our general lack of resource congestion since the 3rd edition.
I see potential for abuse, especially in light of how I see warping, and this may not warp the book, unless there's consensus about that, too. It's a slippery slope, a possible avalanche, so I am passionate.
Quite simply, because this effect does what Teaching cannot, and it does it better. It allows two people to learn from one source. That is otherwise impossible for Arts. Further, Teaching requires some amount of effort (sacrifice) on the part of the Teacher, in that the teacher doesn't learn his material while he's teaching, the student's do. He gets exposure to teaching, or perhaps the material or the language, but that's not much. And then, there isn't any penalty to learning when two people use this effect to study from a book. It makes a resource less scarce. Consider the Bonisagus who takes two apprentices simultaneously, and then just sits them in front of books with these devices, flaunting the requirement that he teach each student once per year. Are apprentices really going to complain that they aren't being taught individually? Are his covenant sodales going to take him to Tribunal and claim he isn't teaching them? How would they really know? Have them read, and then use them in the lab for three seasons, it's tough to say what is really going on in his sanctum.
It's a cosmological shift, unless this device has some really powerful restrictions. Warping is one, simultaneous access is another.
Warping...is a key part of the discussion, and why I believe it doesn't affect the book is because of the following description of warping:
So, is the book affected? IMO, no. The mirrors, perhaps are affected.
Are those within the area of effect of a 6th magnitude (or greater) Aegis affected? Again, no, although it could be argued they are affected whenever a spell from outside is resisted or weakened by the Aegis, but that's a bit of a stretch for me to get there, it's more like the example in the same paragraph, characters living in a flying castle. Or you can go to the next paragraph where it just flat out excepts Aegis of the Hearth.
The Incantation of Lightning and Ball of Abysmal Flame fall within that realm of Brief Effect and High Power, and they are indeed affected by the effect, but really, those two spells, the Warping is incidental to the purpose of the spell. And, IMO, the Warping should affect the Target, not the target, or perhaps the vessel which holds the magic, but in this case, those are very brief and temporary effects, and warping is moot in this case.
Same with Hermes Portal and the like, technically people moving through the portal are affected, because they are relocated from point A to point B, but we have discussed that Portals are superior to spells like Leap of Homecoming or spells using that base with alternative Targets/Ranges (Touch range, Group targets) which would inflict warping...
I dunno, it's something that probably needs to be discussed more in depth.
My interpetation (for what it's worth) that a spell like Eternal Repetition of the Bottomless Pool is quite different from something like Ball of Abysmal Flame, in that the actual target of the spell is the book. The spell creates an image of the book, effect establishing a link to it via an Arcane Connection. Without that connection, there can be no image.
See this in the same way as you would see a spell such as Haunt of the Living Ghost CrIm(In) 35, which allows you to project an image of you to a distant location.
(Let's ignore for the moment that thi spell is R:Per, and that the magus would invent it to be attuned to him. Say this is a R:Touch variant.)
In that case, you are not affecting the individual himself -- you are creating an image of him. Yet, the spell is affecting that person, in my opinion. You can't cast the spell without having a "source" person for the image: that makes him the Target of the spell because you need him to cast the spell. So he would suffer Warping from such a casting.
Same thing for the book. As the source of the image (through the AC) it is the actual Target.
Just M(NS)HO.