Adult fiction is very varied, and includes novels, thrillers, romances and science fiction (sci-fi) to name a few examples.

I like reading novels.
I'm reading a really good thriller.

novel is the most general of these terms, meaning, according to the Cambridge Advanced Learner's Dictionary, a long printed story about imaginary characters and events

The word novel can be modified to make compound nouns such as spy novel, crime novel etc.