Depends what I'm interested in. If there's a movie I've seen that's based on a book that has an interesting premise, I look for the book. Ready Player One; say what you will about the movie, or the book, or both, but it's an interesting gimmick, so after I saw the film, I read the book, and it's better than the movie, even if there's problems with the book too. I also like Roman history, specifically the late republic: Spartacus, Caesar, the Roman Civil War, the early empire, so sometimes I look for books about it, whether it's a straight up history book or historic fiction.
I started reading the Percy Jackson and the American Gods books because it was about ancient gods in the modern day, that's always an interesting idea.