I think that the accepted definitions of what marks a book as a certain genre are too broad to be useful. SF does not necessarily involve ray-guns and time travel (although it might), fantasy does not have to feature goblins and spells (although it might) and Westerns do not particularly have to feature horses, gun-play and stetsons (although they might).
In the Jasper view of genre, Sirens of Titan is not SF but philosophy, anything by Terry Pratchett is not fantasy but satire, and True Grit is not a Western but a “coming of age/epic revenge/historical fiction”.
— Jasper Fforde Guest Post in Locus —“Genre, Speculative Fiction and the Cradle of Ideas”