MASH* by Richard Hooker was very good. In an unusual case of the transfer of a book to a movie Altman used it virtually as his script. MASH Goes To Maine,* as a continuation that cashed in on the success of the movie, wasn’t all that bad. The subsequent books (they were legion) all sucked really, really bad. I have no idea why I read at least five of them.
So did Douglas Adams. Somewhere in The Salmon of Doubt, he talks about Mostly Harmless in an interview, and he says that he wrote it at a terrible time in his life and felt that as a result it came across as very dark and bitter rather than funny. He even says that he felt he owed his fans a sixth Hitchhiker’s book, to finish the series better.
Unfortunately, he died before he could do it.