It seems to be popular lately to take up a series where a deceased author left off. Eoin Colfer took up the reins on this one. He is most known as the author of the Artemis Fowl series of children's books.
This book would be the continuation of Douglas Adam's Hitchhiker's Guide series. It literally starts up exactly where "Mostly Harmless" left off. Rather than summarizing, I'll just say that I feel that it does a very good job of picking up where Douglas Adams left off. I can definitely feel that the humor isn't nearly as natural as when Douglas wrote, but it still made me smile throughout the book.