Douglas Noel Adams was born on March 11, 1952. He is known as the author of the so called "Hichhiker's Trilogy" (5 books, by now) and the Dirk Gently's n-logy (n=the current number of Dirk Gently's books). He is even more known as the author of the impossibly long names of his books. :-)
If you want to write him, you've got three possibilities: e-mail him (I don't reccomend it), send a message to alt.fan.douglas-adams, or write him to the address:
Maggie Philips
% Ed Victor Ltd
121 Wardour St
London W1V 3AT
United Kingdom

These are his most famous (and best) books:
The Hichhiker'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

Dirk Gently's Holistic Detective Agency
The Long, Dark Tea Time of the Soul

And, of course, some links:
The Douglas Adams Worship Page COOL!
Great links page!
His bibliography
Another page
And another one...
And another one...
The Hichhiker's Guide
Restaurant at the End of the Universe
The Page at the End of the Web