Mostly Harmless
1992, ISBN 0345418778) is a novel by Douglas Adams, the fifth book of the Hitchhiker's Guide to the Galaxy series.
The title comes from a joke early in the series, when Arthur Dent discovers that the entry for Earth in The Hitchhiker's Guide to the Galaxy consists, in its entirety, of the single word "Harmless". His friend Ford Prefect, a contributor to the Guide, assures him that the next edition will contain the article on Earth Ford has spent the last few years researching—somewhat cut due to space restrictions, but still an improvement. The revised article, it transpires, will read "Mostly harmless."
Spoiler warning: Plot or ending details follow.
Mostly Harmless picks up the story of Arthur Dent some time after the events in So Long And Thanks For All The Fish. Fenchurch, Arthur's love interest in the previous book, does not appear, her absence being explained away in a single paragraph. The plot revolves around the concept of parallel universes (though according to the novel, they're not really parallel universes at all) and the introduction of a new, sentient version of the Hitchhiker's Guide which can perceive, and act upon, all possible universes at once. The book finally fulfils the promise made at the very start of the series that the fates of Arthur and the Guide are intertwined.
The book ends, as the first book began, with the earth being blown to bits. For good, this time.
Being the fifth book in a trilogy it wasn't clear that this was the concluding book in the series until Douglas Adams's death on May 11, 2001 made it mandatory.
Although the complete destruction of every version of the Earth in every possible timeline, along with the death of nearly all the regular characters, would seem to make a continuation extremely unlikely, Adams had expressed an interest in continuing the series, remarking that the afterlife-enhanced state of the regulars merely meant he would not have to waste time at the beginning of the next book gathering them together or explaining what they'd been up to in the intervening period.
