What is the most unique book you have read?

Let's discuss unique books. What is the first one that comes to your mind? The uniqueness can be about the book's plot, structure, ideas, themes, topics etc.

For me such a book is Cat's Cradle by Kurt Vonnegut Jr. It's been a while since I've read it but I remember how mindblowing it was for me. Every time I thought it couldn't surprise me more, it actually did. There's a bit of everything there: peculiar characters, a substance called ice-nine that can freeze water on contact (as humans are around 60% water, it can instantly kill them), a fictional religion called Bokononism based on the idea that human life has no absolute truth or meaning, a tiny island nation with its own bizarre world. All of these come together to show us a satire of the absurdity of humanity.

As for a unique book structure, I have to say it's Cain's Jawbone by Edward Mathers. It's a kind of puzzle book where pages are not in order and you need to arrange them in order to guess whodunnit. I haven't finished this one yet but it's a cool challenge.