Yeah I've thought it would be cool if you could "learn" things in the game by finding books (rarely) in certain buildings, like libraries and sometimes in residential buildings. It would allow more sandbox features to be added to the game, like foraging, repairing, first aid, etc. It shouldn't involve "skill levels" though, because that would encourage the "upskilling by grinding" pattern that RPGs use... it should just be "knowledge" that's hard to find, but if you find it it you have it, no need to have levels of it. This could also promote more co-operation between strangers and less senseless killing. If some people may have valuable knowledge everyone would be more inclined to talk to each other to find out and there'd be more ad-hoc teamwork. Also you'd be far less likely to become a bandit and risk your character dying if he's learned some pretty useful things. It's hard to implement without breaking plausibility though. Your character would need to spend a fair bit of time reading before he can learn something, so if you just make it so that he opens a book and he gains knowledge instantly, that breaks immersion. Maybe something like you have to put the book in your inventory and you only receive the knowledge if you leave it there for a certain amount of time. Then it starts getting messy.