Time travel books are one of those genre-bending subgenres of science fiction that could easily be fantasy. It's certainly Speculative Fiction. Time Travel Science Fiction features stores where time travel is possible. This could be traveling into the future or past. This is a fertile science fiction subgenre that's been tackled by many of the finest writers. This subgenre shares a lot in common with Alternate History and Parallel Universe subgenres of science fiction, with Time Travel concepts sometimes touching on either.