Overview
Mostly Harmless is a 1992 novel by Douglas Adams and the fifth book in the Hitchhiker's Guide to the Galaxy series. It is described on the cover of the first edition as "The fifth book in the increasingly inaccurately named Hitchhikers Trilogy". It was the last Hitchhiker's book written by Adams and his final book released in his lifetime.
Title
The title derives 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 word "Harmless". His friend Ford Prefect, a contributor to the Guide, assures him that the next edition will contain the article on Earth that Ford has spent the last 15 years researching—somewhat cut due to space restrictions, but still an improvement. The revised article, he eventually admits, will simply read "Mostly harmless". Ford had written an extensive entry covering life and recreation on Earth, but the Guide editors cut it back to "Harmless". Later in the series, Ford is surprised to find that the entry on Earth has been updated to include all of his original material, prompting him to hitchhike across the galaxy and reunite with Arthur on the alternate Earth in So Long, and Thanks for All the Fish.
Adams on Mostly Harmless
In an interview reprinted in The Salmon of Doubt, Adams expressed dissatisfaction with the tone of this book, which he blamed on personal problems, saying "for all sorts of personal reasons that I don't want to go into, I just had a thoroughly miserable year, and I was trying to write a book against that background. And, guess what, it was a rather bleak book!"
Plot summary
Arthur Dent plans to sightsee across the Galaxy with his girlfriend Fenchurch, but she disappears during a hyperspace jump, a result of being from an unstable sector of the Galaxy. Depressed, Arthur continues to travel the galaxy using samples of his bodily tissues/fluids to fund his travels, assured of his safety until he visits Stavromula Beta, having killed an incarnation of Agrajag at some point in the future at said planet. During one trip, he ends up stranded on the homely planet Lamuella, and decides to stay to become a sandwich maker for the local population.
From Wikipedia (CC BY-SA 4.0).