I LOVE LOVE LOVE Hitchhiker's Guide.
Probably one of the greatest Sci-fi/comedy books ever written. (Okay, so by Mr. Adams's admission, it's a "trilogy in five parts".)
If you have the Omnibus Edition (black leather-clad, all five volumes in one), you'll get the bonus story "Young Zaphod Plays it Safe".
Also, you may be interested in the official sequel, And Another Thing... that "fixes" the ending of the fifth book. I haven't had the time to tackle this, but it's on my bookshelf waiting for me.
It was written with the express consent/authorization by the Adams family.