I’m firmly of the opinion that books can’t be ruined by their sequels. I believe each novel exists, untouched, in a vacuum and that no sequel can tarnish the way you feel about its predecessor.
Or at least, I used to think that. Then I encountered some sequels that were (to use the technical term) very bad. With their bizarre plots, unnecessary twists, and character arcs devoid of all sense, these sequels spit in the face of the original novel and forced me to question everything I thought I knew.