Assumptions for this model:
Characters and NPCs are more likely to have average statistics, so lower Characteristics are more common.
People with high/exceptional communication are more likely to write books. [We'll say... doubling value]
People with low communication are unlikely to write books. Also, low quality books are less likely to be sold. (Maybe more likely to be given as a gift.)
Authors with Good Teacher are more likely to write books: Let's assume 20% of authors have Good Teacher, and that people with Good Teacher have higher
It's better to have a formula that feels good and is easy to use than one that is statistical.
I'm assuming no book improvement from Covenants.
Com Books GT books
Com +0: 40
Com +1: 20
Com +2: 10 8
Com +3: 5 6
Com +4: 3 4
Com +5: 2 2
Q6: 40%
Q7: 20%
Q8: 10%
Q9: 5%
Q10: 3%
Q11: 10%
Q12: 6%
Q13 4%
Q14 2%
Obviously, this is unrealistic for what you'd see in a library - the high tier books are hoarded, the low tier books are discarded, and the middle tier are where all the book sales are. We'll get rid of Q6 and Q7 books,
As far as books you can find easily for sale, if you only care about the quality, I would probably settle on the following:
20% Q8 20% Q9 20% Q10 20%Q11 10% Q12 05% Q13 05% Q14
If you are playing with book improvements from Covenants, I would pretty much give every book being actively sold a +1 Quality, since there's no reason not to splurge on high quality ingredients if you're selling books.