The seventh book in the Artemis Fowl series by Eoin Colfer.
SOMETHING IS SERIOUSLY WRONG WITH ARTEMIS FOWL. When he gathers the fairy elite to show off his latest invention, it's clear that the former criminal genius has become delusional and paranoid. He doesn't even trust his closest friends.
Artemis has the Atlantis Complex, a disease that is causing him to lose his mind. And things get worse when a deadly attack is launched against an underwater fairy city. The fairies are desperate.
So desperate, that they need the ruthless, calculating mind of history's wickedest teenager. Will the real Artemis Fowl please stand up?