Since my character is going to write his first input on the topic of magic theory as a Bonisagus fellow, and because I hope this will be his biggest achievement at the time, I intend to have the roleplay around it quite nice and fun. Rewards are always more fruitful when the roleplay around them is smooth and prepared.
So I was wondering: okay the character will write that encyclopedia about magic theory after so many years spent in the lab, experimenting again and again to increase his comprehension of magical theory. That book will be a summa level 10.
But how many tomes will that be?
I checked Art and academe, for real books inspiration but the authors (of A&A, not the "real ones") seem not have used a consistent rule. From Pruscian and St Augustine, I infered a value between 1 book for 3 xp to 1 book for 12 xp towards sum of levels.
If I remember well how we dit home,
tome for a art summa : 1 per 5 levels (so summa level 10, would be 2 tomes)
tome for abilities summa : 1 per 2 levels (so summa level 10 would be 5 tomes)
we could also say
1 tome for 15 xp whatever... which could be more logical (I mean tome by xp and not by levels)
It's arbitrary. I'm listening to a book now about a guy who manages to fit a play by Shakespeare into a nutshell by writing it very small on a single piece of parchment.
You might also base the number of tomes on how good a communicator the author is.
Take the total number of xp the summa can provide, and divide by three times (choose whatever multiple you prefer) the base Quality (i.e. the Quality before skilled craftsmen and resonance bonuses are added). So an Art summa of L12 and a base Quality of 6 would be spread on 4 tomes.
Another (simpler) way to determine the number of tomes would be to say that an author writes one tome each season he works on the summa. So a Com +2 author with a Latin score of 5 writing a Level 18 summa needs 3 seasons to complete his work, thus producing 3 tomes.