What's the Best Book You've Ever Read About Space Travel?

Last week my co-worker Eric Limer recommended The Martian, a book about an astronaut struggling to survive after being stranded on Mars. It's been way too long since I read a book about interplanetary exploration, and it's fantastic to be back.

The Martian isn't just great because of its compelling storyline—it's great because it deals so closely (and entertainingly!) with the chemistry, mechanical engineering, and botany involved with Mars exploration. And I'm raring for more. So I'm curious: What's the best space-focused novel you've ever read? Suggestions, comments, and criticisms welcome below.