Often I ran around with this book in public and people though I was reading the bible...it's a bible of some sort, in a sense.
The Hitch Hiker's Guide trilogy-in-five-volumes is not all on the same level of amusement and good plot-weaving but it's a classic and a masterpiece nonetheless. It's very hard to create a whole fictional universe that's credible and interesting, and just for that Adams's Guide is art in fiction. Each of his characters in believable, endearing and captivating and, eventually, the plot all together in brilliant though the last couple of books show a somewhat decline in the rhythm of the early two books.
Still; it's a classic, it's a must, it's wonderful!