Wait up, there's....
The Hitchhiker's Guide to the Galaxy
The Restaurant at the End of the Universe
Life, the Universe and Everything
So long, and Thanks for All the Fish
Mostly Harmless
... what else?

Those are the ones I have. Also there are the two Dirk Gently books (Dirk Gently's Holistic Detective Agency and The Long Dark Teatime of the Sould), the Meaning of Liff and The Deeper Meaning of Liff, and Last Chance to See...My mother also gave me "Don't Panic" for my birthday one year.

I agree with other favourite quotes posted above and I also like the reference to "a liquid that tasted almost, but not quite, entirely unlike tea." How many times did we all have to work that one out in our heads to get if it did or didn't taste like tea?