If these books are so clearly science fiction or fantasy, why are they regularly misshelved?
As mentioned above, many people believe that general fiction books are better, in some way, than genre books. Some authors take this belief to heart and insist their books are not science fiction or fantasy, despite the evidence of their own words. Publishers think more monetarily and see that, generally, mainstream books make more money. It is also possible that the author, editor, or publisher realizes that some of these books may not do well in the genre section for another reason. Genres, especially science fiction and fantasy, have rich histories and novels that choose to ignore that history can look foolish. As John Clute notes, “any SF reader who looks at [P.D.