The Salmon of Doubt culls together numerous essays he wrote, interviews, a couple short stories, and bits of an unfinished novel. The essays range from autobiographical to musings on technology and all contain his trademark whit. Adams himself mentions how much he was influenced by Monty Python and you can definitely see their shared sense of humor.
The most tantalizing bit of the book for Adams fans is the unfinished Dirk Gently novel, The Salmon of Doubt. Through the assembled chapters, we join Gently on an investigation where he doesn't know who hired him, so he decides to follow somebody at random because he figures he was hired to tail somebody. It's a great start to a story that this fan wishes Adams could have finished.
The Salmon of Doubt is probably only for hardcore Hitchhikers (is there another kind?). It's a fun visit with Adams and a reminder of how this world could sorely use his brand of mayhem.