This book, the fourth in the increasingly inaccurately named Hitchhiker's Trilogy, is, hands down, the best. You probably wouldn't think that were true from reading some of the reviews on this page. However, I was astonished and amazed by what this volume had to offer.
For starters, if you read Douglas Adams just for the zaniness and offbeatness of it all, you may be disappointed by this novel. While those elements are not absent, they are severely toned down for this installment. The amazing thing, though, is that Adams manages to mix in his humor at all with a very touching romance and somewhat serious quest of rather epic (rather than episodic) proportion.
The best part about this novel is that it virtually almost entirely features Arthur, and that's it... at least out of the main characters. Ford shows up a bit, and Marvin is in the last chapter, but Zaphod and Trillian are missing, but don't worry, it hardly matters. Adams more than makes up for it by introducing a marvelous character named Fenchurch, who becomes a love interest for Arthur. A love interest for Arthur? Yes, you heard me correctly.
This book, in my mind, establishes Adams as a serious heavyweight. The levels of humor, romance, irony, wonder, and adventure are consistently high throughout, and one never detracts from the other. Besides, we finally get to take a really good look at Arthur (who had been shortchanged in the last two books), the most human character I believe I have ever encountered anywhere, and we get to see a bit of the earth, which Adams makes us realize is rather a funny place in itself.
Do not miss out on this book. Please. Read it for Arthur. Read it for Fenchurch. Read it for the Rain God. And definitely, definitely, read it for the most wonderful love scene ever written. Besides, if you make it to the end, you'll be rewarded with God's final message to His creation, written in letters of flame thirty feet high (quite the tourist attraction). It's worlds above all the others.