Quality 15 is exceptional and very high even for a good library.
Consider, base quality is communication+6 which gives you 6-11 if you have positive communication. You can get higher if you write lower but considering how high those art books are for scores, I am not sure about it.
You should probably make it varying quality of 9-12 and used saved points for a tractus or two that might be special. There is another thread on here about standard books [url]https://forum.atlas-games.com/t/covenants-standard-texts/593/1] that probably should be your base and then add some exceptional books and a few tracti on top of that.