So Long, and Thanks For All the Fish, the fourth book in the Hitchhikers Guide to the Galaxy series.
This is point where the series jumped the shark in a huge way. The fifth book sucked aswell, but the fourth was especially infuriating due to the sudden and egregious shift in tone. Instead of surreal intergalactic adventures with humorous sci-fi concepts, the book takes place entirely on Earth and focuses on a dull love story. The romance is pretty much the entire book, with only the last chapters touching on anything remotely Hitchhiker-like. And the worst part is that the entire plot is essentially meaningless since the love interest introduced in the book isn't even in the next one!
Y'know, now that I think of it, the fifth book (Mostly Harmless) might be even worse. "Thanks for all the Fish" might have been boring read, but Mostly Harmless is like a straight up fuck you to fans of the series, and ends things off at a terribly. Hitchhikers Guide to the Galaxy really should have stayed a trilogy.