Okay. A review without a synopsis or, in fact, a review. If you're on this page, if you're reading these words, then you've heard of Hitchhiker's and you want me, as a total stranger, to tell you if it's any good or if you should buy it or not.
Just buy it. Don't ask questions. Just buy it. And read it.
This book is a must-read for anyone interested in (1) Sci-Fi, (2) Humour, (3) PG Wodehouse. The Hitchhiker's Guide To The Galaxy, with its sequels, stands in a genre of its own. It has been compared to other iconic British comedy such as Monty Python material, or such tongue-in-cheekers as Dr Who. While these share a history with Hitchhiker's and its author, Hitchhiker's has its own unique contribution to make to contemporary entertainment and contemporary literature.
No need for a synopsis or a gushing review here. You can Google hundreds (if not thousands) of websites, some fan-driven, some professional, that will tell you all you need to know (and much that you probably wish you could avoid). And you can always see the fair-to-middling 2005 Spyglass Pictures movie for the general plot (with significant studio-pleasing alterations). But nothing can compare to the almost real thing: The Book.
Almost real, because the real real thing is the original and unsurpassed BBC radio series (1978-80). You can still buy that one on CD. If you want to experience the majesty of Hitchhiker's in the medium its episodic and staccato plot was originally penned to fit, then get the radio series. Otherwise read the book(s).
Why? Because I said so.