Grasshopper Jungle, by Andrew Smith, walks the line between fantasy, science fiction, and complete and utter insanity. It takes place in our world, but our world featuring giant, apocalyptic praying mantises, nuclear explosions, and two-headed babies, among other things. The hero, Austin, accidentally unleashes the giant praying mantises upon the world and has to try to clean up the mess while also navigating his romantic and sexual feelings for both his girlfriend Shann and his best friend Robby. Because it straddles the line between sci-fi and fantasy, I might not have included it here, except that it's an deft treatment of teenage bisexuality, and also it's so thoroughly weird that it defies easy categorization and so I can get away with including it in a list of fantasy novels.