You work in a store that presumably wants to make money.

A property like "Fight Club" is so well known and tied to the movie, that the store would be better off keeping its organization tied to the movie more than anything else. Because there's a better chance that a person who knows the movie will want to read the book than a Chuck Palahniuk/Horror reader not finding the book due to "incorrect" genre placement.

Give this one up. Your need to be technically correct will not help the book get in more people's hands. If anything, the difference between the movie and book will be an interesting surprise for the reader.