Artemis Fowl by Eoin Colfer

Twelve-year-old criminal mastermind Artemis Fowl has discovered a world below ground of armed, dangerous, and extremely high-tech fairies. He kidnaps one of them, Holly Short, and holds her for ransom in an effort to restore his family's fortune. But he may have underestimated the fairies' powers. Is he about to trigger a cross-species war?