Not to point out the blindingly obvious but Hitchhikers was an adaption of a radio play, not a book. The book was also an adaption of the radio play, as was the tv series, and technically parts of the radio play were adaptions of both the books and the tv series as well as early drafts for a movie adaption of the radio play and bits of the tv series as well as some content from the books and........ well........ It's still the best five book trilogy in the world. Adams died too soon.