The Opal Deception (Artemis Fowl, #4)

The evil pixie Opal Koboi has spent the last year in a self-induced coma, plotting her revenge on all those who foiled her attempt to destroy the LEPrecon fairy police. And Artemis Fowl is at the top of her list.

After his last run-in with the fairies, Artemis had his mind wiped of his memories of the world belowground. But they have not forgotten about him. Once again, he must stop the human and fairy worlds from colliding—only this time, Artemis faces an enemy who may have finally outsmarted him.