Books about alternate dimensions, specifically a parallel universe impinging upon our own or vice versa. The typical story might be about the discovery of technology that allows one to travel to another parallel dimension. Or on the flip side, others from another dimension invade our own. This is a rich sub-genre that's been mined heavily by science fiction writers. It's also one of those genres that can see a merge between fantasy, science fiction, and horror.
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.