Mostly Harmless was described on its cover as "the fifth book in the increasingly inaccurately named Hitchhiker Trilogy".
Earlier in the series, 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".
© Haydn Thompson 2017