Science fiction, for too many people, equals trash.
Unfortunately, there are plenty of books on the shelves that support that very expectation. Each year publishers bring out dozens of mass market paperbacks and even hardcover books that are the equivalent of the 1950s pulp magazines, with their bronze bra-clad warrior women of Mars on the covers. But amidst that sea of space opera and dragon-filled quests lurk some excellent works of literature. Science fiction has evolved since its early days of H.G. Wells and Robert Heinlein, and it has evolved well.
For a moment, let us pause to define genre. Science fiction isn't the only genre fiction. The term is generally considered to include romance, westerns, action adventure, mystery, fantasy--theme fiction, if you will. Everything else is lumped together as "mainstream," and within that broad category, we have literary fiction. In every genre, but most frequently in science fiction with its more flexible boundaries, some books fall into a nonspace between genre and mainstream.