Here's a snippet from an article by Peter Gill which appeared in the British newspaper The Guaridian on Feburary 3rd, 2011:
"As the book's title suggests, Adams, like most authors, was not afraid to borrow, and there are revealing similarities between Welsh's Hitch-hiker's Guide to Europe and The Hitchhiker's Guide to the Galaxy. One of these provides perhaps the most intriguing explanation for "Why 42?". As you may remember Adams had Deep Thought perform a little expectation management and say: "You're really not going to like it" before revealing the Ultimate Answer.
Curiously, Hitch-hiker's Guide to Europe had told of visitors to the UK searching for family roots finding "the answer a little disappointing" – after travelling around the world in search of "the solution to the most puzzling question of all". A coincidence, perhaps . . . but this coincidence is on page 42."
But I like this answer better: "The answer to this is very simple. It was a joke. It had to be a number, an ordinary, smallish number, and I chose that one. Binary representations, base thirteen, Tibetan monks are all complete nonsense. I sat at my desk, stared into the garden and thought '42 will do' I typed it out. End of story."