What do they actually do? Me and my brother are playing aveyond and used a book with Lydia, it changed nothing. It gace her no spells and didn't change any of them.

Are we doing something wrong or what?

Lydia can learn spells from it if it's equipped. She only learns at uneven level number though ^^

You have to equip the spell books on to her as shields. She'll then be able to learn the spells from them when she levels up.

Thanks! She must've been an even level