Why 42?

So, I think you are interested in main Adams' works (clicking on these links you'll be moved to Amazon.com site; buying a book will help answer.42 to keep up the good work):

The Hitchhiker's Trilogy (5 books, all from Ballantine Books):
The Hitchhiker's Guide to the Galaxy - $7.50
The Restaurant at the End of the Universe - $7.50
Life, the Universe and Everything - $7.50
So Long, and Thanks for All the Fish - $6.99
Mostly Harmless - $7.50

Most recent works:
The Salmon of Doubt: Hitchhiking the Galaxy One Last Time
Ballantine Books - 2003 - $11.16

Dirk Gently's Holistic Detective Agency Pocket Books - 1991 - $6.99 The Long Dark Tea-Time of the Soul Pocket Books - 1991 - $7.99 Last Chance to See with Mark Carwardine Ballantine Books - 1992 - $11.16