It may behoove you to first read my review of the previous book Life, the Universe, and Everything.
There are trilogy purists—my high school biology teacher is one of them, as I recall—who ardently insist that the Hitchhiker trilogy should never have gone past three books; that the final two are utter rubbish, unsuited to the title. I don’t remember much about books 4 and 5 from when I first read them, many moons ago, but after rereading So Long, and Thanks for All the Fish, I’m inclined to agree with.
You might recall that even the third book, Life, the Universe, and Everything, began to approach a critical mass of incomprehensibility. But it, at least, was a genuine space opera, and one could easily find Adams’ jokes. So Long…, by contrast, is 90% set on a reconstituted earth, where Arthur Dent falls head over heels in love with an English lass named Fenchurch, and we are completely unsurprised when she reciprocates. What about Trillian? Adams later dismissed her—and by proxy, the previous three books—with a sentence or two. Thanks, Doug.
I would even be OK with the poncey love story if it were merely one facet of the book, but that’s all the book is. There are little snatches of Ford Prefect in some random galaxy, and again at the end, and dent finally goes back into space in the last chapter, but really, Adams could well have just called this book Arthur Dent Gets Laid (and other running gags). The book is a vehicle for running jokes from the previous books, without introducing new content; it’s the Jay and Silent Bob Strike Back of the Adams canon.
Dolphins are an intelligent species that left prior to the earth’s destruction? Been there. Fenchurch is the minor character mentioned in the paragraph at the very beginning of the trilogy and promptly forgotten? Check. A lot more of the whole “flying is forgetting to hit the ground” joke? Check. Token Ford Prefect one-liners? Check. Conclude the book with a minor joke/plot point from the previous novel for some weak sense of continuity? Check.
Really, what a load of rubbish. Why did Douglas Adams ruin a perfectly good trilogy with this? It’s like the straight-to-video sequels to famous movies that have none of the same actors and 1/100th of the budget. I’m reading them because I’m a completist, and maybe you’ll feel the some way, but if you’re a casual Adams fan, I’d stick to the first three and not bother.