I would be cautious in changing the rules for book writing. In 4th edition, the formula included Scribe skill in the quality of the book. As learning included Concentration of the student (and Intelligence too), players could combine skills to become better writers and better students. In the end, the best way to become a covenant of powerful magi was to quickly learn Scribe and Concentration (and the one who gets to be excellent in Scribe writes good books about Scribe ability, studied by the one who, in the meantime, got excellent in Concentration, who in turn writes good books about Concentration, and everyone could quickly become excellent writers and readers, which really boosts arts and abilities learning after that, considering that the other magi learned something interesting they could write about in the meantime).
The current rules do take skills in consideration : the author must be at level five in an art, or two in a skill and he must be at level five in the language used. Not written in the core rules, but in the covenants book, the notes must be given to a skilled scribe (then an illuminator, a bookbinder) to have a usable book.
If think that a very high level in bookbinder, scribe and illumination would not change quality (where quality means clarity); a book is well bouind, well written, well illuminated or is not.
Perhaps you could add to the basic formula a bonus equal to the lowest score between Teaching and (used language - 5) ? Or fix a level in Teaching and Language to allow a bonus to quality.
As for tractati, I only allow them if the author made a discovery (in twilight or in experimentation), one tractatus in only one ability or art per discovery.
I allow to write "parchments" (i.e. papers) in unlimited numbers, but those books are limited to the level of the author and the readers could not go over it when studying.
You could also resurrect "libri quaestionum" from fourth edition (with a target level but higher quality).
The idea of using extra seasons to refine a book is good, I gladly borrow it!
Don't forget glossing of texts for having a +1 quality.
You could imagine a system where the author is not himself a so good writer, but collaborates with a better writer who is not so skilled in the subject art or ability. Yiou use the Communication score of the skilled writer (note the so-called author). Then you could have high quality with high levels (and no need to change the system), without escalation in book quality.