Pardon me for possibly being dense, but why do we think that the magi are so jealous with their findings?
I guess there are three perspectives: those of the writer of the book, the owner of the volume to be memorized, and the magus to memorize the volume.
Perhaps magi are averse to people reading their books, but where I come from, writers like to be read. Especially academics, whose lives are largely spent in the pursuit of prestige, want their works to be read as widely as possible. So most writers will want their books to be copied, memorized, and so forth, and so wouldn't have any objections to people using the memorization spell. There might be exceptions â€” a magus comes up with a spell or insight that he only wants to share with his closest friends â€” but then the volume is unlikely to make it into a library.
The magus to memorize the volume obviously wants to memorize the volume.
Now how about the owner? Well, the owner might want to become famous for having the best library, and so be jealous of people copying his books. That is, it's possible that the Order librarians have a problem with the dissemination of knowledge. (Mind you, that strikes me as very unlikely, but we'll go with it.) But, since neither producers nor consumers of books will be interested in librarians' prestige, they'll collaborate to get around censorious librarians. I imagine that the Code would contain instructions requiring that all books be available to anyone who swears by the code, thus thwarting the librarians. People might even cast spells on their books, making them self-combust if anyone ever tries to prevent anyone in the Order from reading, memorizing, or copying them.
If spells like this are possible, then physical texts become much less important. A magus could carry around the whole library in his head, refreshing the spells periodically. Instead of reading the physical volume, he would reflect on his perfect mental image. That would take just as long as studying from a physical book, but wouldn't require a library.
Here's an alternative. Maybe knowledge is dangerous.
Since a whole lot of the books that people would want to memorize and/or copy contain mystical formulae with great inherent power, whenever you write a book, there's a spell you cast on it that's sort of a parma magica that keeps magic from interacting with the book. That way opening the book doesn't let demons into the world, or whatnot. (Some of the rules for Jewish texts, in one of the old Kabbalah book or the new Book of the Divine, would lend themselves to this way of thinking.) The mental photocopier spell would have penetration of the copyright spell built into its very structure, and that might add, oh, I don't know, three levels of magnitude, thus making the copy spell level 55 and a ritual. That might make copying a book magically sufficiently difficult as to ward off attempts to do it by any other than the greatest and most powerful archmagi.
For the same reason, having an entire book in your head might be destabilizing. Perhaps the mind can only handle so much internal magic â€” memorizing even a fairly large number of spells might not do it, but memorizing entire books might. Maybe the ad hoc limit to build into the spell is that the memorizer gains a warping point per memorized book per moon.
That said, the level 40 version of the spell seems like a pretty effective deterrent already. But if we really want to keep physical volumes important, it seems like we might want to go beyond just making it bloody hard to magically copy books â€” we should make it virtually impossible.