Something like Pillars 1/2 where the more you fight them and learn about them the more info you have on each creature is a wonderful idea. I did plan on creating a thread about it too but got distracted, whoops! lol. Either get info through fighting and ofc as you said, from books and such.
In the same vain, an encyclopedia. All the lore about deities, locations, etc that you come across gets added on that.
