While "Life, the Universe and Everything" is not quite as good as it's two prequels ("The Hitchhicker's Guide to the Galaxy" and "The Restaurant at the End of the Universe"), it is still a very good book and very worth reading. This time, Arthur Dent, Ford Prefect, Slartibartfast (the creator of the norwayan coastline) and Zaphod Beeblebrox go out to safe the universe, a very humerous adventure.
To completely understand this book, you should read "The Hitchhicker's Guide to the Galaxy" and "The Restaurant at the End of the Universe" first!