I've been reviewing Art of Memory in TMR, and it got me wondering. Can you use the spell Memory Palace of the Sage to learn books that are useful beyond just learning. Two examples come to mind, but I'm sure there are others out there. The one I can think of are: (1) the Book of Solomon and The Keys of Solomon used with Ars Notoria, and (2) a numerologist's book, per the Hermetic Numerology mystery.
As for the Book of Solomon and the Keys of Solomon, a practitioner of Ars Notoria needs them to recite the various rings, and they serve as a Q20 tractatus in the Art Notoia ability. Could a magus use Memory Palace of the Sage to memorize the Book of Solomon and the Keys of Solomon and simply recite the rings from memory?
As for a numerologist's book, the text of Hermetic Numerology talks about "consulting" the book. Could that consultation be mental via a book memorized using Memory Palace of the Sage? I understand that the book is opened, seemingly at random, at which point the magus inspects the relationship between words on the open pages. But it's that word "seemingly" that gets me. That tells me that it's not really random. The text also says that the user's attunement to the book allows it to practically open by itself. Could that be the same with a memory of that book? Could the magus use that memory to effectively open to a seemingly random page on his memorized text?
I personal don't think that you can get magic properties of a book into your memory place and with this I think the Numerology book don't work.
But i.e. for someone having Gematria the Torah would be a "normal" book with nearly endless usage.
For the Ars Notoria, I'd say yes. Physical possession of a copy of this rare book is less important than the ability to understand it. One could theoretically simply have a number of backup physical copies scribed, the book isn't inherently possessed of magical powers as I recall, though it does contain great knowledge.
For the numerologist's book, I'd say no unless the magus also possesses Inscription on the Soul. A numerologist's book normally requires vis investment, and part of it's power comes from the ability to open it to 'seemingly random' pages and gain insight therein. It is a spiritual, as well as a mental object - this is represented by the vis required to invest the numerologist's book. On the other hand, if the magus wanted to combine some of the numerological book talisman features with inscription on the soul, with the idea that the 'book' is stored within their memory palace, I could get behind that.
Worth noting that in either of these cases, a single application of a spell like "Loss of but a moment's memory" (or other Mentem effects) that targeted memories of either book could irrevocably damage the knowledge contained within.
Another note, these sorts of individually specialized, significant magical texts also seem to be prime candidates for the following warning in the spell description for Memory Palace of the Sage: "Additionally, rumors exist of texts that contain traps against such memorization, even fatal ones, and of the dangers of memorizing a text written by an incomprehensible Criamon or an author close to Final Twilight." For someone without proper exposure to the mysteries of numerology or prior initiation into the understanding of the Ars Notoria, attempting to memorize these texts could/should be big side effect material.