Once upon a time there was a struggling writer for the British TV show Dr. Who who was good at writing funny prose and not particularly good at writing plots. He had an idea for radio show, and it was something of a hit--enough of a hit that he turned it into a BBC TV series and a smash science fiction trilogy of five books or so. Riding that wave of popularity, he dusted off some of his old Dr. Who scripts and wrote a couple of more books that were undoubtedly better written but didn't do as well. Little he did after that was particularly popular, and it was a shame.
The radio show was, of course, The Hitchhiker's Guide to the Galaxy; this is also the title of the first book based on the series. I don't know if anyone's reading it anymore, but certainly most people with any taste for science fiction have read it; like Monty Python, it's been a fruitful source of catch phrases and in jokes ("42!" "I'm so depressed!"). In short, it's the story of a man named Arthur Dent, whose planet is destroyed to make way for a hyperspace bypass, and his attempts to make it home again. It's genuinely funny, the sort of book that you can't read to yourself--you have to share the funny bits with everyone around you. The next book in the series, The Restaurant at the End of the Universe, is equally good. The remaining books decline fairly continuously, until the final one, Mostly Harmless, was widely judged to be "Mostly Awful". If you've not read the first two or three, though, you should.
Before descending into that nether pit, however, Adams wrote two unrelated books, about an unorthodox sleuth named Dirk Gently: Dirk Gently's Holistic Detective Agency (Dirk always tries to solve the whole crime) and The Long Dark Teatime of the Soul. These books are better plotted, and somewhat less funny--the gag is no longer king--though they do have some very good bits, such as Dirk's duel with his refrigerator. Their primary flaw is Adam's penchant for spending lots of the book showing you what various characters are doing without naming or explaining them, rather like I did in the first paragraph of this page. It's a reasonable tactic in a mystery novel, but he carries it rather too far.
Don't Panic, and the original radio scripts are also available.has written a book about the Hitchhiker's phenomenon,
If you like this...
..., you might also be interested in' Discworld, 's Castle Perilous, and 's Myth series.
Books by Douglas Adams
The Hitchhiker's Series
The Dirk Gently Series