How common would books be on the subject of education be in the Ars Magica setting?
As common as you'd like them to be.
The line doesn't have any stance on the availability of any books.
Arthur did some work a couple of years ago when I asked the question about how may books exist and their qualities, when I asked the question about what the average quality was...
Are there RL books on the subject?
There are. best start with
then follow up on
and en.wikipedia.org/wiki/Priscian .
I'd just point out that if someone is has a lot of experience with Teaching, in most cases (stuff like the Gift being an exception) transmission of this (and other) experience is best accomplished by ... Teaching, rather than by writing books about it
Socrates was considered one of the greatest Teachers of his age, perhaps the greatest, but did not write any books.
Nor did Pythagoras or any of the Pythagoreans.
Xenophon and Plato did that for him, thereby conveying their esteem to us.
But they weren't the Good Teachers that Socrates was.
We could go on ad infinitum over who was the best or most qualified, but the general take away is:
Not all good (or great) teachers wrote books.
Though Plato's works were more on philosophy than teaching, though to be fair teaching wasn't considered a thing to be studied and written about until the Roman empire, so the first question would be what survived from that and the second question is what if anything has expanded upon it- both of these are YSMV questions. Some volumes of Quinilian's works apparently contained instruction on education, and he was considered a master of the field.
And there is really no reason, why magi should not anticipate Poggio Bracciolini's rediscovery (see en.wikipedia.org/wiki/Institutio_Oratoria) by 200 years, and thus add a useful book to their covenant library.
Or there could be copies yet to be destroyed in a covenant library somewhere. Just because Poggio Bracciolini discovered the works in 1416 in a pile of rubbish doesn't mean there weren't copies in use in 1220- it is doubtful that a pile of rubbish held manuscripts that had been there for over 200 years.
Which would have required a discovery of the institutio oratoria by some Hermetic magus or covenant librarian before.
Such as the library that they were found in, before they were rubbish.
There I a difference between "needs to be discovered beforehand" and "not lost yet"- the second is certainly a possibility in 1220.
Additionally the issue is irrelevant- Quinnillian also inspired numerous other authors, so it is not as though this was the only summae on teaching- it simply demonstrates that they did exist and that the study of teaching as a field began after Socrates, which may be a large part of why that particular great teacher didn't write a book on teaching. It is not as though the entire concept of books on teaching hinges on whether Quinnillian has been re-discovered or not.
Ahm - with books this is just a matter of perspective.
In the years after the Church fathers, the appreciation of Quintilian declined, and his voluminous books were often taken apart. The institutio oratoria didn't disappear, though. Somebody just had to change the lack of appreciation. In real history, this was Poggio's merit - not his digging up a copy in a derelict room of St. Gallen. In Mythic Europe, this could as well be a magus.
I would expect that a large part of a text waning in ars magica would be due to cheap copies- If Quintilian's work started out as level:6 Q:14 summae, after being copied quickly about 5 generations(to gain three times as many copies) the newest copies would be level 6 quality 9, and there would likely be newer works with higher value, and at the same time older copies might be degrading from usage or poor storage, so in 1220 there are probably quite a few low value copies of Quintila's work along with more modern texts on education.
There's this fellow who is a fan of Constantinople and its history. He pointed out that before the Sack of Constantinople, it's only the ignorant West that needed to "discover" them. Which means that either there were no magi that far East, or none of the classical greek literature was lost.
That's a major discrepancy between culture cloned from MA Europe and culture evolved from a few century of East-West exchange.
IIRC this is also a topic in subrosa #16 865 AD Voventes Centennales: as the Eastern Jerbitons use Classical Greek in Hermetic Magic, they can also use the libraries of Constantinople to the fullest, and work on Platonic and Aristotelian Magic Theory (A&A p.11).