Ringworld by Nevinyrral. The sense of scale is just… otherwordly. Sorry, couldn’t resist.

Non-fiction wise, I’ll have to go for The Fabric of Reality by David Deutsch. That guy is a true living genius. Also, I like how the book gets VERY entertaining towards the end, unlike every other non-fiction which basically invites you to skip the last 33% because you don’t need to read it.