Don't Panic is sort of a biography of Douglas Adams. But it concentrates almost exclusively on The Hitchhiker's Guide to the Galaxy, and the events and circumstances in Adams's life that contributed to the creation and development of the Hitchhiker-phenomenon. It was originally published back in 1988, then revised in 1993, and now revised again, after the untimely death of Adams.
Don't Panic is written in the same style as the Hitchhiker novels, and is at times quite amusing. There are no less than 31 short chapters (the first one is chapter 0) in the book, dealing with various distinct subjects. This is followed by five appendixes. There are a lot of direct quotations from various people surrounding Adams, and from Adams himself, but there is never any information given about when or where these people said these things, which is a major drawback.
There is a lot of trivia here, but a lack of depth, and the book does seem to repeat certain myths (for example, the well-known one that Adams first got the idea for Hitchhiker while lying drunk in a field in Innsbruck). Still, if you're a fan of Adams, this is essential reading.