What's the last book you read where, if asked whether you thought reading it had been a good way to spend your time, you would not just say "Sure, I think so," but "Hell yeah!"?
@scott My answer probably is of little help to people who aren't math nerds, but it is the HoTT Book (a mathematics text, and there are actually caveats about which sections I read, which ones I skipped, and whether it was the read itself or all the mathematics I learned spurred by the book as much as the book itself).