I like spellbooks. I like to imagine all the notes, marginalia, and non-spell contents in them. To spur that kind of detailed artifact on the fly in a game, I've made a generator for random spellbooks and their miscellaneous contents. Each generated spellbook will have 20 items listed. If you have an idea for how many spells you want the book to contain, simply stop after it lists that many.
This is a brainstorming aid; it will give you prompts for spells, usually pretty actionable ones, but it will be up to you to designate the particulars.