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.