An option to add or remove libraries, sure. Not to toggle between NO, OLD_SYSTEM, NEW_SYSTEM.
I will try to make it as balanced as possible, thats what the thread is for. If I make Masterpiece books have a 1% dropchance, you get one in 100 reactions. If I make the free-skills from a library to go to lvl 3, thats a big difference from lvl5. If I make the reactions require skills (reading or student, a social skill), then they would take time to run. If I make the reactions to only add +1, till a max of 3, instead of instantly to 3, then it would take trice as long. If I make the reactions need a journal, or ink+quill, or ink+paper, or a notebook, or any reagent, suddenly the entire balance changes.
If masterpiece books train to legendary, but are used up... thats different from "train to lvl10 in one skill, but are preserved".
I can do sooooooo many things. I just have to figure out which one is the most fair, while being useful in the game, and interesting to the player... as well as easy to understand. Real simple,