The Hitchhiker's Guide to the Galaxy by Douglas Adams
This book is also one of my favourites.
Basically, it follows the life of one Arthur Dent after his house and then his planet are demolished and he discovers his best friend is, in fact, an alien.
This is a truly hilarious book. Adams's wit is dry and intelligent, and the characters – Arthur, Ford Prefect, Zaphod, Marvin and Trillian – are beautifully created and stay with you long after you put the book down.
If you haven't read this book yet, do so.
I can't wait for the film to come out next year!