What’s your favourite fantasy “non fiction” book?

I’m talking fantasy books that aren’t narrative necessarily, but are presented as informational books set in fantasy worlds. Like encyclopaedias, history books, “science” books (especially with diagrams) or my personal favourite, art books. I’ve even seen some cool cook books. Whatever you think fits!

Doesn’t necessarily have to be a companion to an existing series, but it is cool when it is.