Best compare ArM5 p.62 Artes LIberales, p.66f Philosophiae, and A&A p.136ff Who's Who in Philosophy to get proposals for the Level and Quality of the books considered Authorities at the universities of Mythic Europe.
Note, that for a text gaining enough prestige to be considered an Authority, it needs more than just good stats: university politics and curricula, and even Church politics, are at the very least as important.
Roots and Branches are qualifications of books on Arts within the Order of Hermes, and there is no need to extend them even to Arcane Abilities: the mere existence of books on Parma depends on the saga, the reputations of books on Finesse, Penetration and the Realm Lores depend on the traditions which use them, and the reputations within the Order of Guernicus' writings on the Code of Hermes and Bonisagus' books on Magic Theory might well still tower over more useful books of later authors.
One of the things I notice comparing the list in A&A to a lot of lists of Art books here on the forums, is that the Qualities of real books are much lower than those of the fictional ones.
This more or less fits my understanding of how academia works - the theoretical upper limits of Quality are rarely if ever explored, and the common reference books are often difficult to struggle through. This is something that would be fun to explore in Ars Magica - I'm just not sure how to represent it.
I think Aristotle can pass as an authority on Logic, but that's only a fraction on an ability in ArM, and thus Aristotle is neither an authority on Artes Liberales nor on Philosophy. The existence of an authority implies a subject which can be traced back to a single source. The only two cases I can see where such a subject corresponds to an ArM ability is theology and magic theory, and the latter is dubious since magic theory has developed a lot since Bonisagus' time.
While the actual numbers, 3/6/9, may be fine, I would object to the very idea of standardising it. As somebody else mentioned roots and bracnhes are about reputation more than quality. A book on philosophy or church lore may very well earn similar reputation, and for the sake of this argument, let's call them roots and branches even though the terms are Hermetic and should be reserved for books on arts in Real Game. Not all abilities have roots and branches, and when they exist, there is no reason they should have the same level or same quality. A branch is the finest book written ever, and that may be a mediocre book if there is little competition.
I would suggest to make all book of excellent reputation unique, and give the rumours as much space in the saga as the book itself.
I meant that fictitious AM characters have a (not quite) Mary Sue quality that real historical characters do not. After all, no one has written any books criticizing Bonisagus!
HoH:S, page 9(box) has a L5Q11 summa on Penetration ("Ultor") by Elaine filia Flambeau, a copy of which is noted to be in the Great Library of Durenmar
Ah. Sorry. Good point ... which may help to explain why we, as players, put so much emphasis on Bonisagus four centuries later. As every magus will know, Bonisagus is hopelessly outdated. Many central aspects of the arts are better covered by other authors, such as enchantments. Using four separate arts instead of Vim is just silly. We, players, never seem to fully appreciate how out-dated Bonisagus is. Bonisagus writings are still excellent of course, especially as sources for the more obscure details of the arts, and essential for those who want to push the limits further, but authorities?!
Ah. Sorry. Good point ... which may help to explain why we, as players, put so much emphasis on Bonisagus four centuries later. As every magus will know, Bonisagus is hopelessly outdated. Many central aspects of the arts are better covered by other authors, such as enchantments. Using four separate arts instead of Vim is just silly. We, players, never seem to fully appreciate how out-dated Bonisagus is. Bonisagus writings are still excellent of course, especially as sources for the more obscure details of the arts, and essential for those who want to push the limits further, but authorities?!