"The Hitchhiker's Guide to the Galaxy" by Douglas Adams. The best Sci-fi novel I've ever read. It has four sequels, and they'll keep you busy for at least three months.

P.S.: I know that this is an old thread, but it's still listed as important, so I figured that it would be ok to post on it (besides I've used the term "groundbreaking" to describe this book a couple of times.)