I started a game with 3 player new to Ars Magica and one came up with the idea of a mirror or glass attached to a wooden frame and enchanted with an effect like the spell "Eternal Repetition in a Bottomless Pool" from the Covenants book.
This will allow any mage with one to read any book in the library without worry the book will not be available.
My question is he wants to use a strand from the registers of each book as the arcane connections needed for the books to display on the glass. I'm not sure if thats enough material for an A.C.
Any ideas for other arcane connections to books? As this item if for my Hermetic Sanctuary idea and multiple readers might be made the library's catalog can't be used.
We use a ERiaBP enchanted mirror in my saga, so I might have some insights that would help. However we do use a HR (actually a pre-play OR) that allows Fixed AC to be made in about a week.
There is no listed minimum material to make an AC from, so any limit will entirely be saga dependent. If you want an AC to be larger in size than the material allows, doing something like encasing it within a sphere of glass or quartz is effective. Almost any clear or semi-clear material could be used. The effect would be like a bug trapped in amber. This would allow small things to have a size of 1~2", easy to work with and much less likely to be lost.
If you specifically plan to make AC to a book, you can manufacture them with something like a sacrificial page which can have parts cut from to make them. Even books without a sacrificial page will normally have a mostly blank page in the front which could have a corner cut from to use. If you plan to fix the AC right away rather than after the book is filled in (for things like a journal) then waste material generated in its manufacture can be fixed. My group does this for the captains logs of our trade ships and as a way for our "agents" to send information.
Another use for ERiaBP enchantments is long range communication. If multiple locations have them and a blank book they give AC of to each other, they can rapidly transmit large volumes of information.
Another use of them is for spying. If you can get piece of a log book or ledger that you make an AC from, you can at any time access it. This actually work better with new ones which have little to no use yet. Easier to get access to and having new information entered into it longer. Heck, if you learn where someone sources their blank books from that is by far the easiest way. My group has done this to spy on other trade groups and merchant houses.
Is there anything disallowing multiple copies of the library's catalog?
If you do not allow it there is an alternative - which will also make the whole thing both more secure and more efficient. Do not make each reader a magic item. Instead, enchant the catalog or something attached to it so that it has an AC to each (mundane) reader, and can sense and react to actions taken on them, by projecting the appropriate set of pages.
Note that magic can produce stuff well beyond modern technology: the "reader" might be a simple token the size of a coin, that produce a full, 3D illusion of the original book.
... which is as close as a cursory inspection came to stating that multiple catalogues are explicitly permitted.
I came up with a similar device for a pbp on the old Atlas forum some years ago. I'd thought of it as a cute non-issue device but the alpha-SG thought it was a big deal. Apparently, he'd expected debates over who gets to read what when, to figure in the saga in a major way.