This is rather weird when it happens that novels are a huge success, and not the movies based on them. Even if you read the novels that are very good and its movies based on them have failed, fans reading them shouldn’t feel ashamed because the movies were awful. you can still read them.
It has happened many times. One of the Janet Evanovich’s novels is an example of it. Her novel One For the Money was made into a movie, and it failed at the Box Office, but the novel itself is a huge success.
I think some successful novelists fall short of their motion pictures. Mario Puzo wasn’t exactly an example, but you can make a case of The Godfather, Part III, movie. It wasn’t a success, but Mario Puzo’s novels are selling in huge numbers.
Rick Riordan’s novels are a huge success, which the fans love his novels. I have read some of his novels and they are very good. The Lightning Thief was a novel made into a movie, and the movie itself failed, but the novel is successful.
The fans should feel happy that they are reading, regardless which its movies are successful, or not. Be happy that you have bought books and are happy to read them to own them. The fans have nothing to do with the success or failure of the authors. You might think that the one side of the authors success is great.