"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.)
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"We can categorically state that we have not released man-eating badgers into the area."

-UK military spokesman Major Mike Shearer