My history with The Hitchhiker's Guide to the Galaxy is pretty long and storied. The short of it, however, is that it's become my very favourite book of all time of all the many pieces of literature I've taken over my 25 years of life.
When I found out that Neil Gaiman had written a history of Douglas Adams and The Hitchhiker's Guide, I knew that I had to expand my education. Of course, the book came out in 1988 (my version is from 2003) so it's a tiny bit outdated, but the history remains the same.
If you were ever interested in learning more about the sci-if comedy, from radio broadcast, to book, to play, to TV show, written by one of the most prolific English writers of all time, Don't Panic is well worth your time. It's written in and Adams-style, full of hilarity and subtelty. Including interviews with the original creators, producers, and actors, you get a real sense of how the story came to be one of the most successful books of all time.