Basically, the stats given to books on Abilities by Ars 5th edition very often defy logic.
The biggest problem is how often books are declared to be a "summa" in an Ability when they absolutely are not.
Look, I am perfectly willing to grant the Euclid's Elements meets the game-rules definition of a summa -- "an organized account of a topic, taking it from the basics up to a certain level" -- if the topic is geometry. But geometry is not an Ability in ArM, and the Elements is not a summa on the Artes Liberales. When the Ability is Artes Liberales, the Elements are clearly a collection of tractatus -- "an in-depth treatment of one aspect of the subject", as geometry is just one aspect of the Artes Liberales. (And yes, I know the Elements also covers number theory, which is arithmetic.)
This problem repeats over and over again with historical works on subjects in the Artes Liberales and Philosophiae, because those Abilities are much broader than any famous historical work. Thus work after work that would be perfectly fine to call a "summa" on a narrower subject are declared a summa on the whole Ability. But while that's where it's most obvious, it isn't actually limited to just those obvious cases.
I mean, it is, I guess, possible that the lost epic poem Titanomachia was in fact a proper summa on both Magic Lore and Faerie Lore (as claimed in Dies Irae), but I expect that it, in fact, was not an organized account of the basics of how faerie and magic creatures/things/powers actually work in Mythic Europe, but a story about a specific group of Magic beings and a specific group of faerie beings. Similarly, the fictional book Pagan Norse Beliefs (TtA) might a summa on Faerie Lore, but it seems incredibly unlikely given the title, instead likely giving in-depth detail on one specific group of faerie beings.
(The same unlikelihood would seem to apply to a book called Further Notes on Magical Notation being a summa on Magic Theory, though, you know, maybe I'm just not getting Janus of Bonisagus's humor.)
The second big issue I have with canonical stats on Ability books is the entire topic of Divine Lore. That we have any books at all labeled as books on Divine Lore, because it is not an Ability of its own, nor an area or organization. And if we assume that they're all actually Dominion Lore, we have the problem that Dominion Lore is an Arcane Ability.
Arcane Abilities are quite restricted in who can learn them -- magi, and people with the rather limited Virtues Arcane Lore, Blood of the Nephilim, Custos, Failed Apprentice, Folk Magic, Master Bard, Redcap, Senior Bard, Student of (Realm), or Wise One. It is blatantly inconsistent with this to have books on the Arcane Ability of Dominion Lore sitting in every monastery, madrassa, and yeshiva in all of Mythic Europe.
Look, I realize the game's Ability list is optimized around the core intended way to play the game, and that the book rules are optimized around providing stat lines for fictional books on the fictional Hermetic Arts. There was always going to be trouble fitting actual historical books into those boxes. But a huge percentage of the officially-published statlines for books on Abilities are grossly inconsistent with how ArM defines books and Abilities.