Its been a while since I've read it but I remember not liking it (the fourth book wasn't terribly great either, but not too bad). The plot (such as it was) goes something like:
Some sort of wierd rift across multiple dimensions has allowed an alternate-reality earth to side into our universe.
Arthur, finding out about this, wants simply to go home and pick up where he left off. He even manages to find his old house.
Back in the universe a faceless evil corporation, which is never really expounded upon to the point that it exists only to be a faceless evil corporation, has bought out the guide and is canceling the expense accounts of guide researchers and generally turning the guide into cubicle-hell.
This upsets Ford.
The best part of the book is an extended sequence where Ford is at the Guide's Main Office being chased by a bunch of guard-robots, its simply brilliant, I can't describe it in this short space.
The Faceless Evil Corporation's plan is to release a new edition of the guide that uses some sort of poorly-explained multi-dimensional McGuffin to contain all the knolwedge that ever has/ever could exist, or something like that.
They bring the uber-guide to earth and something goes wrong, I forget what exactly, and earth and all of the alternate earths blow up plus Arthur, Ford and the rest of the cast (except Marvin who, if I'm remembering correctly, didn't even appear in the last two books).
The whole thing left me with the sense that Adams was tired of the series but the fans wouldn't shut up so he was going to end it in a way that left no possibility of more sequels. It was like he wanted to say "Look! I blew up the earth, AGAIN! And not just the earth but all possible earth's. And Arthur too, he's dead, do you hear me, d ... e ... a ... d spells dead. You remember that thing from the last book? About him having logical proof that he couldn't die until that one thing happened? Well I threw that in too, on the last page, very sudden like so you would know it was real and couldn't complain. The series is over so stop calling me, you're filling up my answering machine."
Never heard that bit about his publisher holding him up for a sequel though. That would certainly explain things. Anyway, yeah, no need to read the last book if you don't want to.
Later.
