Discuss precautions regarding securing Apollodorus's Book of Control and then resolve things in character.
(( Do I get to participate in this discussion, since Stultus arrived after Apollodorus' death? ))
I would say certainly, that this would be an ongoing discussion, subject to revision and what not. I just want a clear understanding of what's there for Cumhachd to circumvent.
Keep in mind, that it (Apollodorus's inner library) is warded against House Guernicus specifically, it may or may not have been warded against Tremere, but given his admonition to not share the book with House Tremere, that's not an unreasonable guess. Too bad Rose as a PC never materialized... 
Let me also add that this is not a permanent loss of the book, it's not designed as a story to be punitive at all[1]. There is something I'm doing here that I need the players to trust that it will work out and be interesting at the same time. It requires players divorcing themselves from their characters a bit. PCs can and should be totally PO'd by what's going to happen. Players should give me the benefit of the doubt, and let the story unfold. That's why I'm asking for reasonable precautions for securing it above and beyond the basics that are known and/or can be reasonably guessed.
Oh, and let me add, that since it has been enchanted and is clarified, it must stay within an aura of sufficient strength or be destroyed. Probably all characters but one will assume that Cumhachd doesn't wish to destroy the book...
[1]Based on my review of the Planner, is that Isen is impacted by the loss of the book, based on the current plans listed in Google docs document. This is a momentary blip, though.
Stultus would suggest the following things:
- The real books are in a deep walled-in niche within the sanctum. The niche is divided into two separate chambers, a front one and a back one. Accessing the niche requires a spontaneous ReTe/MuTe spell; accessing the back niche requires emptying the front one, and casting another such spell.
- A number of other books (spare lab texts, vain books, or even blank books), re-bound to resemble the real books, within the front niche.
- Optionally, a second set of fake books on a hidden-but-easy-to-find shelf within the sanctum.
- A register of the real books, within the Princeps' sanctum.
- A separate Arcane Connection to each book, held by the Imperator.
What's the design of said spell?
I said, "spontaneous". Any spell designed to bypass a wall would do. MuTe (change mortar between the stones to get rid of its "sticky" property): Base 1, +1 Touch, +1 Diam, +1 Part, +1 to affect "stone": level 5. If you have trouble with Muto or Terram, go get Fiona.
Alternately, hide the niche behind a tapestry and have the blocks of stone that cover it (and blocks of stone that compose the inner wall) be mortared together, but unconnected to the rest of the wall, requiring ReTe or a strong enough mundane to remove. That's not really the important part. The important part is that the outer niche contains false books, and most people once they find something 'hidden' don't look any further to see what might be hidden behind it.
Well, the design of the spell, be it spontaneous or otherwise is actually really important, because as you say, just "go get Fiona." But, I submit that it is an argument a player makes, and not one a character makes. Spontaneous spells of fifth level require CS of 25 to be cast without fatigue, and also no flaws which inhibit casting spontaneous magic. Fiona certainly fits the bill. But, reasonably, do you think she would appreciate or other magi would rely upon her for access to the book? And the alternative, of casting it oneself is risky due to the chance of botch. And that design suggests that it weakens the joints, but it doesn't move any components of the wall, which means it becomes a Rego spell, with a Muto requisite, so you have to go get Isen, or someone else who is proficient with Rego. Even if the Muto is "free" it's a huge imposition to a lot of people a lot of the way around. A Tytalus might be willing to live with this, for a time...but eventually the risk of 1 botch per season, on average to access the book is a bit too high for most magi. And I figure magi are smart enough to know that pushing themselves to cast a spell 90 times in a season during the course of a day is probably a bit risky... Especially for those magi with a Gift that makes spontaneous spell casting more difficult, or the characters with Flawless Magic....
And that's fine, as described. It's within the ballpark of entirely reasonable, whereas I submit the idea of needing a "spontaneous" spell is not. Now, perhaps this incident might give the council pause and they would consider inventing the spell, and then ensuring everyone knew it, but, even if we go back and revise recent history to allow for the creation and dissemination of said spell, I doubt it will have been implemented, yet.
As Jonathan mentioned in the other thread, Tranquillina knows Ward Against the Curious Scullion and is certainly happy to use it on behalf of the covenant. She casts it with a typical Penetration of 28 or so (in the regio aura). Of course, that could hinder us as we go about our own business - that's a call we have to make.
I note also that the hardest part about guarding a book (or anything) is guarding it against magi who know what they're looking for. Stultus's suggested subterfuge will work against people looting at random; barring access with spells and walls keeps out mundanes. But so far Tranquillina herself could probably bypass all suggested defenses - teleport in and out, boom. Is there anything reasonable that can be done against that sort of intruder?
Why cast it 90 times a season? If you're actually studying it, you remove it for that season and keep it within your Sanctum. Give it back once you're done.
Why would you want to cast it without fatigue? Again, if you're only casting it twice a Season (four times, twice for each of the walls).
Why wouldn't you use ceremonial casting? We could keep a set of casting tools that give, say, a +3 bonus, within the sanctum, in a drawer.
But yes, ReTe is the better option.
I will also say something that the player knows from his IT work. Security is inversely proportional to convenience, and there is no such thing as perfect security. You come up with a workable compromise, that's all you can do.
Besides, this was just what I came up with off the top of my head. Other people may have other ideas.
I challenge Tranquillina to teleport (blindly, without an AC) into a 2' by 2' by 2' niche filled to the brim with books. 
It's not an actual room, it's a space big enough to hold the books.
Stultus' suggested defenses are actually specifically to keep out magi who know what they are looking for, by putting fake books where they could be found. If a magus has all the time in the world to look around, yeah, they'd find it. But if they're in a hurry, they may be deceived by the first or the second fake copy.
That is also why his defenses are not magical in and of themselves: because magical defenses will show up on an InVi spell, and because There's Always Someone Better (tm) who can bypass them magically. Tranquillina's Penetration on "Ward Against the Curious Scullion" is 28. Do you really think, say, Bilera's or Valerian's Corpus resistance is worse than that? To say nothing of Cumhachd.
Because if you remove it from the location warded against Guernicus, you remove a major protection...?
See above, I think that the magi would not reasonably remove it from where it is, and would study it on the lectern on which it sits.
Again, you can do all that, and if it gets the CS to such a level that all the magi could reliably cast it without exerting themselves, I'm on board with it. Some magi, won't agree to such a stipulation, though. And I'd be hard pressed to accept any and all players saying their magi would accept it.
That's exactly right. And security is a process, not an endpoint.
Oh, I'm not trying to be a Negative Nancy, I'm just trying to come up with reasonable precautions taken. A method requiring a spell is a reasonable precaution. Some magi wouldn't mind casting it spontaneously, but even going back and revising character sheets/history for the creation of said spell, and then the dissemination of said spell, I don't think everyone would be there, yet. And then it becomes a question as to when everyone would sign off on it being effectively off limits to mundane intrusion, keeping in mind it already is with Ward of the Curious Scullion...
Okay, that's a fair point.
Re-quoting what I said, so people don't miss it in the storm of posts...
I agree with Jonathan's comment that the place where the book is stored must also be a place where the book will be studied from.
I'll also take on your use of "blindly" 
Seeing through a wall into the room beyond is trivial: a CrIg spell to create light inside the room (by touch from the outside), then Eyes of the Cat, then Prying Eyes - all level 5 spells.
Who wants to place their grogs here? One player, one grog...
I'm thinking the same thing.
I was working on an item ( a brick) that would give the effect of the Diagon Alley opening and closing. I'm also thinking of a linked trigger that would accept a metal command. I think a base of 5 could understand/listen for 2 words. Or, if you really really wanted to be tricky, an emotion.
Junior is more of a henchman thug. But Roberto has been trying to train him, so maybe some guard duty will do him good.
So, before any of this happens, just the moment before, Junior looks over at the other guard and says
Wanna armwrestle? Looser buys drinks after the shift ends.
"Sure." We'll assume that there's a table nearby, and chairs. The other guard, who shall identify himself eventually, maybe, stands and let's TB and Junior get to it.
This is just a few days after the fight in the bar, so Junior isn't healed, yet, but he's coming along nicely. "How are the ribs?" He says it out of genuine concern.
The two get down to arm wrestle. And TB smiles. It looks like Junior's going to win, but then he says, "You remember how you said I was using magic the other day? And then I said, I wasn't? Well, that's not the case today." Junior finds himself frozen in spot, with his arm halfway to a pin, but his entire body is immobilized.
TB then stands, turns to the other guard, and smiles.
Alexei suddenly appears outside the chambers with a burst of a somewhat spciy hops aroma and takes a second to look around. Brionne was standing at his post like he should be but the other grog was sitting at a table with his hand in the air. That was odd, and why wasn't Brionne saluting him?
Alexei raises his staff and sees someone that is vaguely familiar at best. "Who are you? Stand fast!" he commands.