I may as well mention The Hitchhikers Guide to the Galaxy. Perhaps it's not a bad book per se, but I never got on with it. OK, it's clever in places and there are some very good, pithy one-liners, but it's most a death march of nerdy jokes and enforced quirkiness. It gets tiresome quickly. By about page 4. The plot is basically random wacky events wrapped like I do Christmas presents. It was sort of the book equivalent of a laughter track left on play. I never tried the sequels. The perhaps worse thing is that it's the sort of book that haunts because people won't shut about it. Ha ha, zorgonic pffazlefaz or somesuch. I say people, but it's always men. Oh, I'm sure there's a rule-proving female exception, but it's a book that channels a peculiar form of nerdy masculinity. It's the book equivalent of the workplace posters that declare 'you don't have to mad to work here but it helps.' The only solution to those requires a flame thrower.
I can't do Pratchett either for similar reasons. And dullness, the couple I've read went on forever. Like the people who read them.
Oh, and Dune. Fucking Dune. It made Tolkein's prose seem zippy. All I remember is a long, long slog through marshy prose, every paragraph tries to suck you down, full of dull italicised monologues that stumbled through dozens of pages and then somehow get up and keep going. The entire book is driven by a homeopathically thin plot, derivative of everything from the Bible onwards. The movie features Sting and is somehow far better without being even close to good.