And here ya go mate, do your thing!
Life and Background
Brendan was born a bit more than 50 years ago, son of a maid and a grog at Elk's Run. Having his affinity with books, sharp mind and desire to learn recognized, he was eventually trained as a scribe, and after a few years Brendan became a personal scribe to Ímar Ua Donnubáin, a necromancer of Praesis, as part of a deal between Ímar and Oswald of Bonisagus, princeps of Elk's Run.
Once in Praesis, Brendan worked hard. Mastering the ability to build a Memory Palace and record detailed information on almost anything, he made himself useful in other ways, gaining the confidence of the Ímar and the other magi and rising the ranks in the covenant, eventually becoming Praesis’ head librarian. He used his Memory Palace to keep track of things both magical and mundane, and to record the deals and arrangements between the Order and the Coil Trí. The library of Praesis thrived under his care, and tending to it was his greatest pride. As far as he was concerned, he was living the dream.
The Siege of Praesis changed everything. His library was pillaged and burned. Ímar passed away. Brendan survived due to sheer luck. He was found by the Irish rescue team and offered a position as Inchmore’s librarian.
Brendan still has stored in his mind the library catalog of Praesis. He also remembers much about the Coil Trí, as well as the key details of the previous deals between the Coil Trí and the Order. His greatest desire at the moment is to restore Praesis library to it’s former glory, but his well of knowledge makes him an asset to the magi which cannot be ignored.
V&F discussion:
Virtues: Arcane Lore, Educated, Custos, Minor Enchantments, Great Intelligence x2, Puissant Art of Memory, Affinity with Art of Memory, Book Learner, Focus Power (???)
Minor Enchantments: a ring with the spell “Enhance the Memory Palace” at R: Touch, D: Moon, T: Ind. (level 25)
Virtue Discussion: Focus Power
Covenants suggests that the library catalog can be used as an AC connection to all the books that are part of a collection (seems reasonable to me) and that it can be used to target single books instead of targeting them all as a Group (seems like a stretch). The three spells that exemplify this are A Simple Charm for the Recovery of Misfiled Books, The Demand of the Frustrated Scholar Soothed and Determine the Location of the Absent Volume, all on Covenants pg. 100.
If we accept that this (targeting single books instead of groups), I'd ask if it's reasonable for Brendan to have has a bit of magic in his blood, in the form of a focus power that allows him to find books which are recorded in his mind's library catalog. In that case I'd likely make him into a Failed Apprentice instead of a Custos. The Focus Power virtue gives him 25 spell levels to design something akin to a formulaic spell, which is just enough for something akin to "Inexorable Search", possibly with a +1 mag for the non-standard effect (capable of singling out only one book).
Flaws: Poor Memory (faces not stored on the memory place), Lame, Fear (of fire), Ability Block (Martial), Driven (restore Praesis library), Covenfolkly Demands (Hibernia and Coil Tri)
Flaw Discussion: Covenfolkly Demands
So, I'm not sure on how to classify his story flaw. As I see it there are a lot of people who would have an interest on his knowledge, and between previous favors and deals with the magi and enemies seeking specific knowledge (or seeking to stop him from divulging knowledge) he will get in different kinds of trouble, political, hermetic, academic, martial... This seems a bit like the Knightly Demands flaw from LoM, but geared towards the demands expected from a covenfolk instead of a knight. Since he is offered little compensation for these troubles and can't really say "no" (due to several different reasons) this is Major, just like Knightly Demands. Does this makes sense?
However, I dislike the name (Covenfolkly Demands, pfff), and perhaps there's an simpler way to describe his involvment in stories. Perhaps just take Enemies and focus on a specific kind of trouble (eg. people who want the deals with the Coil Trí to fall apart)?
Hooks
- Recovering the lost/stolen books from Praesis (includes negotiating new copies of books he knows were part of the collection)
- Ímar's Research: Brendan often helped Ímar with his research, and would like to see it completed as a tribute to the magus. He has a bit of knowledge about the things Ímar was working on. Part of it relates to the catach of Elk's Run.
- Coil Trí: Negotiating (or renegotiating) deals with the Coil Trí would benefit from his presence, and it would be hard for him to say "no" for these knowing he is particularly suited for the job.
Mind Palace
Vague ideas: Still deciding between his mind palace being in the shape of Praesis library (each memory is a book, or a series of books) or a more traditional one.
With Puissant and Affinity we are looking at an Art of Memory of 11+. With Intelligence 5 he can record even lengthy pieces of information quite easily, and he would have 55+ locus where to store the memories. I think it's best to list a few, but not all of them? This way we can later say things like "locus 34 is still undefined, right? Well, let's say it's a memory of a particularly heated discussion between magi of Praesis and the Coil Trí, because the SG has an idea to spin around that" or "let's make locus 49 the precise and complex steps to enter a regio where Ímar had a secret laboratory" and etc.
This point regarding Arcane Connections stood out to me. I normally think of Arcane Connections being based on Sympathy of composition (a part has sympathy to what it formerly composed (and potentially vice versa)) but it seems clear both that this example doesn't fit that mold and that we have examples (ArM5 pg 84) that are not based on direct composition (though this is most common).
One might argue that a book that had been part of a collection for a sufficient period of time might form a connection to the rest of the collection. And/or that a catalog or book register used to track the books thus might accrue such a connection much akin to a frequently used object or tool. This would seem an example of connection due to close association and use rather than composition. The duration of such a connection however I think would likely tend to be "days" or "weeks", perhaps up to "months" but I note the examples in the "years" grade of connection are all examples of direct composition.
If Brendan was the librarian and he used his memory palace catalog/book register to meticulously track all books in the collection ... in theory it might have as much of a claim to such an associative connection as a paper register. And he might have something of the same degree of connection as the librarian, arguably. Yet these connections wouldn't last very long beyond the fall of Praesis even if we granted them a "months" duration.
Though, in the spirit of "yes and", if we grant that such a connection could exist it would also be possible for it to be fixed? I'd be curious for opinions. But that would open up the possibility for a complex (perhaps even magically created) memory locus to exist that is also a fixed connection to the former books in the Praesis collection.
Seconded. There are a few rare exceptions in the books. The two on the top of my mind are the library catalog and the usage of a basin of water on the spell "Enchantment of the Scrying Pool". I vaguely remember the usage of a city charter as an AC to the respective city, but I'm not sure this one is on the books - it might be an idea from the forum.
From Covenants, p.98:
The ritual that binds libraries to their catalog was invented during the second century of the Order’s history. A magus performing it requires a season and a pawn of Vim vis. (...) This ritual is the ancestor of the ritual than fixes Arcane Connections.
So I think it's possible for it to be fixed, but it's not given (being a precursor might mean that it's not quite as effective)? I'd lean to yes, but I'm biased.
There are other instances of characters having fixed ACs stored in their mind (the one which comes to mind is the virtue Homing Instinct from RoP:M), and I was thinking of that, but a ritual to fix it would also work... But it would require a new ritual when new books were added, I presume?
(By the way, going with the description of Homing Instinct an AC which is an artifact of the mind cannot be given to others, which would prevent a magus from using Brendan's mind catalog to simply teleport the books back).
Still, the thing I'd like to focus on before that is the fact that there are spells in Covenants that use an AC to a Group to target Individuals of that Group, which is not something I'd usually allow for hermetic spells (but could allow for powers and other non-hermetic effects on a case-by-case analysis). Are we ok with that? If we aren't, then I don't think this AC would be much useful anway.
Rule of cool in my book (get it, library, books? I'll show myself out now...), so I'm totally fine with it personally speaking.
I think it's potentially easy enough to explain away as non-hermetic magic that we can't explain/understand? So again, I'm totally fine with it. @Vortigern ?
I think it significantly depends on the nature of individual Arcane Connections.
Our usual examples of Arcane Connections that are Connections of Sympathy of Composition (as previously discussed) would be ACs that directly connect to one Individual target. I would generally think that with such an AC that you could target both that Individual (or whatever) and any larger target size that also included that individual/part etc. If you have an AC to a person you could hit them with a Group effect, affecting whatever Group they happen to be in at that moment, and that should work fine. Much like Intangible Tunnels can ground effects out from what they link to. You have an AC that was a rock taken from a particular room in a castle? You could throw a CrIg through and create fire at that spot, using Individual. But you could also use Room to affect that place. Or go up to Structure to affect the whole castle. All are effects grounding out from the original AC, but must include the original linked/connected to thing which the AC lets you reach out to.
However this is not an analogous Arcane Connection to the example of the library catalog/book register. This seems something born out of both/either association/use or the aforementioned special ritual. (I think there seems to be some meta-intent of the authors to try and prevent books being used as ACs back to libraries?) The point however is that this explicitly seems to be an associative AC that does connect to the whole group in the same way, either as a group or individually, at the same time.
To compare this to another example of a different sort of associative AC we might say that, in theory, a military unit could march with a given battle standard (the legion's eagle for example) that is always used by them and strong associated with them ... and that this could thereby become an AC to the group in a similar way. This would seem then to be an AC that could be used to target individuals or the group in a similar way to the library catalog/book register, because it has the same sort of associative connection to all members of the group and not only one of them.
Thoughts?
I was thinking of exactly the same example (well, with a registry of the soldiers instead of the banner). It seems natural that if it works like that for books, it would also work in this situation.
I'm not sure I'd like all associative ACs to be capable of either targeting the Group or the Individuals (but hey, your game), as this would mean you effectively have an AC to several different things. I'd rather clarify it to be something specific to the ritual (mentioned on Covenants) used to bind the books to the register.
Thus, the battle standard could generally be used to target the whole unit as a Group, but not particular individuals of said group (unless a similar ritual was invented for the battle standard-soldiers).
Worth mentioning that a book catalog has a list of the books, a battle standard does not have a list of the soldiers who march under it. This is important because casting spells require awareness of the target. Since on the catalog each book is clearly and unambiguously listed, I think it makes sense to say that there is awareness, even if I have no previous contact with the library or it's books. But using a standard, how do I single out a particular soldier? Do I need to factually know that someone is a member of the army?
Furthermore, could I single out a group within the army (say, only men, or only blondes, etc) and target them through the battle standard? What if using a register where each soldier is named? What of the book catalog, could I target only books bounded in leather, only the ones written in paper, or use other screening methods?
TBH I regret asking. I think I've opened a bigger can of worms then I thought at first. 
If it's all the same to you all I think it's better we bury the subject deep, where no light will shine upon it, and leave it for the future. We have other things to deal with after all.
Setting aside discussions as to what the canon answer is, I tend in general to lean towards a more conservative approach to ACs as well. They seem greatly more proliferated in canon than I would prefer.
We could significantly simplify the issue and future explorations of ACs by adopting a more restrictive interpretation of them. The question is how restrictive then. I would almost prefer to adopt a strict ACs of Composition only view, removing ACs of Association, as it would seem like it would remove the vast majority of any potential problem cases. And by nature mean each AC is only linked to the thing it formerly composed a part of.
I do think targeting ( even with ACs) requires being able to "declare" a target to some extent, but how specific that needs to be isn't clear. And at least with ACs of Composition, Intellego effects appear quite capable of telling you what something is linked to. (True Lineages, pg 72, The Whole From The Part)
The ritual that fixes connections between a library catalog/book register and the collection seems like something that would make sense viewed instead as a unique breakthrough based on the ritual that fixes ACs (rather than a precursor). This would allow it to function differently/uniquely and require something that functioned similarly to be a similar breakthrough research project.
Edit: Part of the intent with this special ritual for fixing a library catalog as a group AC seems also the effect that it prevents a book from the collection being a link to the whole collection (that being then only achievable with the catalog). This seems to be a desire to prevent the use of "associative ACs" of the books to be able to target the whole collection, from a gameplay perspective. Though restricting ACs of association might address this concern more directly/generally.
Thoughts?
I was waiting a bit to see if anyone else would jump in. Since no one has:
This leaves out things which I would say are quite canonically ACs, from a storytelling POV (such as clothes or personal belongings). I'd rather not see those going. But I think if we axe connections to multiple things at once, things should be fine.
We could go a step further and establish that a connection must point to a single Individual (capital I), but then we jeopardize connections to Rooms, which are cannon (unless the Room is also a single Individual/Structure). TBH I don't think we would lose much.
This has already become my headcannon after this short discussion, so I would be in full support of this approach.
I don't think it would work like that? A book would be a connection to the library itself (the physical location of the library), not to the other books?
I think the intent of the ritual is to have an AC to the books w/o the need for taking and fixing a small piece of each book.