Science Fiction and Horror can overlap to create a scary story where the source of fear is another planet, alien race, interstellar travel, a plague or something else from the Sci Fi genre. This is a highly popular sub genre when it comes to pop culture, especially fodder for movies and video games. Think movies like "Aliens", "The Mist" and "The Thing" or video games like Deadspace.
Speculative Fiction is the term that incorporates anything where stuff occurs that's out of the ordinary -- explained by either supernatural terms or scientific terms. As such, Speculative Fiction covers Horror, Fantasy, and Science Fiction. It's also a catch all term used to categorize books that are not easily classified into any of the genres, especially literary or slipstream works that are experimental form. The books on this list are simply a selection of a few works. See the others lists for more specific works