Piranesi is unlike any other book I've read. I'm not sure if I would exactly put this under the Fantasy genre; if anything, it's a lot more subtle and perhaps more in between Fantasy and Magical Realism. It has incredible world building, such a unique story, and for me it was the kind of book that I couldn't put down.
It's an interesting story about alternate universes. I also like that it makes you think — what would happen if I had made different decisions in life? Am I happy with the decisions I've made? What are the things important to me?