As Andrea says below, teen books are great ffun to read for adults as well as teens. As additional prooff, I offer you The song of the Quarkbeast, by Jasper Fforde. Fforde, who has written several series for adults, started a series for a younger crowd with The last dragonslayer. In this sequel, you will find light spheres that run on sarcasm, additional references to marzipan as a controlled substance, and an enlightening and thought-provoking view on how trolls view the human species (on page 200), as well as the most delightful sentence I've read recently.
"She was so crabby, in fact, that even really crabby people put their crabbiness aside to write her gushing yet mildly sarcastic fan letters."
The song of the Quarkbeast