If you want to wing it, but with some sort of idea behind what might possibly exist, this topic had some interesting discussion on what is possibly available as far as quality and and the numbers involved. Arthur also created a spreadsheet which allows one to change assumptions a bit.
It's an interesting exercise to figure out what is possible in your saga. I'm leaning towards swapping the columns for Spell Mastery and Arts, personally. I think texts on Spell Mastery generally receive short shrift, and think certain spells would have dozens of mastery texts available (PoF, BoAF, IoL, AotH) because of their utility. Any commonly cast ritual[1] in your saga should also have many texts available, and I can see mastery texts as being a good way for a budding writer to get his foot in the door of Hermetic writing so to speak.
[1]I believe that ritual casting always requires a stress die, and has botch dice at least equal to the pawns of vis used, mastery only reduces the number of botch dice based on the mastery score, and doesn't eliminate it entirely. This was heavily and heatedly discussed here. RAW is, at best, contradictory, but it's pertinent here, because it explains why I think there should be more mastery texts available...