From my view, with Covenants rules, it seems that a good "best" book in a particular Art would have a Level somewhere in the 16-22 range and Qualities ranging between 12 and 18. Good primers run Level 6-8 with maxed out Qualities -- though Level 10-11 books should be relatively common with acceptably high Qualities.
Something to keep in mind, and hinted at in earlier posts: If you have too many excellent or high level texts in your library, your magi will not get out or do anything important until much later in thier careers.
(Hmmm, study a Quality 16 Level 15 summa or go vis hunting/bargain with the local fae for collection rights/go to Tribunal/whatever -- how tough a call is that?)
My advice: make the library a source of stories and you will be greatly rewarded.
One thing that has worked VERY well in a saga I play in is to make the library skewed and seriously deficient in a number of key Arts and Abilities. It forced a few magi -- and one I would have never guessed, but should have -- to go out looking for good texts (political and "resource" stories), build good relationships with other covenants (good repeating story hooks there), and travel to retrieve the texts in question (battled a horde of low Might demons on that one). Oh, and make enchantments that let you get the summa back in decent condition!
(some background reasoning follows...)
Now, given that, our covenant has at least a modest Level summa in every Art and a number of really good ones (one is a Level 22 Muto text on loan) plus some really good ones in "scholarly Abilities". We're still weak-ish on tractatus but those will continue to fill in as we continue our development.
As for general guidelines, it all depends on how many "publishing" magi there have been in your vision (and your troupe's vision) of the Order.
My personal view is that no more than 20% made it thier mission to write any summa -- call it 400 in the life of the Order to date -- and of those MAYBE 40 had/have enough talent in the topic AND the writing ability to make it worthwhile. I know, that's pessimistic, but I've chosen to view the idea as indicative of most magi wanting to be very good at magic and not make OTHERS very good at magic. So 15 Arts, maybe 10 broadly important mystical/academic Abilities and suddenly you have just maybe 3 or 4 GOOD summae on, say, Ignem and 1 just-good-enough summa (L15Q12) in Herbam (written by Magister Neville of Loch Leagan who also wrote a damn fine summa on Parma!) with a just a few L6Q10 Ability summae (Parma, Magic Theory, and possibly Penetration OR Finesse being the likely candidates).
Now, again in my view, the Order is filthy with tractatus and lab texts and the trade in both is brisk since (a) a tractatus takes only a season to write and an older magus can write a lot of lab texts and (b) there are so few summae.
A more collegial and scholarly Order would, in my view, produce more and higher quality summae, probably the same tractatus and lab texts, and somewhat less pwerful older magi since they are spending more time writing and less time accruing power.
For example, Maga Rowan of Merinita has a skill of 44 in Imaginem (990 XP). She has an affinity in that so lets assume that after appplication of the +50% modifier, she was able to put 15 XP a year into that -- she's also Driven -- so that takes 66 years. Let's tack on 10 years of time where for whatever reason (Twilight, Mystery initiation, politics, etc.) she was not able to work in her Art. To get all the bonuses to Quality (i.e. "make it the best text on the topic in the Order") she and her scribe and bookbinder and apprentice will probably have to spend at least a year or more probably two writing and constructing and maybe even enchanting the text. The text itself wouldn't be ready until 78 years post-Gauntlet. Best case is that she's 98 years old and probably older.
True, you could REALLY skew her to where she completes such a wonderous text sooner, but then you would have to somehow convince me that she's OK with having a poor spell selection and/or most of her other Arts below 7 or non-publishing mystical Abiliites around 3 or less. (although, in my model, she also has 300 levels in spells and enchantments)
All this is to say that a Level 22 Quality "wow" summa is a very rare bird indeed. It would be very hard for me to believe that there are more than 8 Level 20+ Art summae at any good-to-great Quality level extant in the Order. And a Level 8 summa in Penetration better have been written by an Archmagus Hoplite Queasitor Imagonnascrewyouupis!