Artemis Fowl and the The Opal Deception (Artemis Fowl, Book 4)

Artemis Fowl's memories of the fairy race have been wiped, and his one fairy ally, Captain Holly Short, is on the run. He needs his memory back--and fast--because a power-crazed pixie is out for revenge, scheming to overthrow the Lower Elements Police. With Holly gone, Artemis is depending on a flatulent dwarf. Things are about to explode.