So. In real life, we have some clues and allusions to works that would have existed. In Ars Magica terms, Artes Liberales, Philosophiae, various Area and Organization Lores... Various religious texts. Probably Summae and Tracti on other various abilities too.
But in terms of Mythic Europe, what was really in there? Assuming no limits, what would you put in there or want in there for a game?
I'd have to say there would be a fair bit of supernatural ability tracts disguised as mundane topics.
The complete Critias, including the details of the fall of Atlantis?
Diodoras Siculus's Books of History, which a small fragment of survived.
Suetonius' Rome, including the Roman festivals and probably all sorts of items a mage interested in the Mercury cult would find useful.
Quite of a lot of mythic and epic poetry; practically any Greek or Roman hero's story would have magical value.
Archimedes On Polyhedra, and On Spheres: Useful for magical theory? Or certain cults?
Ctesias' On Pneumatics would be of some value to Verditius magi or other engineers; Memorabilia could be interesting to Bonisagus magi.
Euclid's Book of Fallacies would be wonderful for Artes Liberales or Philosophy, or both.
Of course, there are also works that are not only lost in text, but also lost in memory. We don't even know they existed.
Wikipedia has a decent starting list under Lost Work.
A lot of precursor magics to Mechanica of Heron (Archimedes, for example) and Fraternity of Samos Cult Lore.