It may to behoove you to read my review of the previous book in this series, So Long, and Thanks for All the Fish
The fact that five books does not a trilogy make is not lost on either the publishers or author of Mostly Harmless. Some printings of the book label it as the fifth installment of the “increasingly inaccurately named Hitchhiker’s Guide to the Galaxy trilogy.” For my own part, I happen to think that series more or less tanked after the third book, and from what I read, the author’s opinion doesn’t stray too far from my own.
Mostly Harmless is a bleak book; it ends poorly, and not at all in the way that such things should. It is also extraordinarily random, harking back past the unintelligible Bridget-Jones-on-acid nonsense that was So Long, and Thanks for All the Fish. In true Adams style, it finds several quantum versions of Trillian (Fenchurch having extraordinarily disappeared for good), Ford Prefect, and Arthur Dent crisscrossing both the galaxy (spatial), time (temporal), and extra-sensory dimension (?) in an odd quest with little explanation, less purpose, and no sort of half-decent ending whatsoever.
Really, it’s rubbish. Even Adams, apparently, despaired at the bleak note with which he ended it. I’m less concerned with the bleak note and more with the relative paucity of humour in the damn thing. A sixth was in nebulous form in Adams notes when he died in 1991, but we’ll never know what it would have been, beyond a few sketchy chapters.
Mostly Harmless, like its immediate predecessor, felt like nothing more than a tribute to running gags: a lot of references to the popular first three books, with a tenuous connecting thread from which Adams attempted to prise a coherent plot. He failed. Sorry, but the book was rubbish. It left me cold on any one of a number of levels, slightly cheated, and wanting to go read something funny or substantial to wash the taste of failed satire out of my mouth.
Really, avoid it. Even for the mere couple of hours it’ll take to get through it, it will only aggravate you and leave you questioning the literary legacy of a dead man. Let it go.