The correct answer really depends on what you're into, and what kind of person you are. There are a lot of subcategories in fiction (just like there are in non-fiction). Lord of the Rings is much more than mere fiction. It is possibly interesting from a linguistic PoV as it contains an entirely new language called Quenya developed by Tolkien. It also contains a lot of subjects which are relevant IRL. Without getting…
Wikipedia tells me that the genre is called Non-fiction novels.