"Artemis Fowl: The Eternity Code" by Eoin Colfer (Artemis Fowl, Book 3)

Artemis Fowl has created the most powerful new supercomputer known to man--using stolen technology from an elite race of underground fairies.

When the computer falls into the hands of an IT billionaire with a mob connection, Artemis is in deep trouble.

Only one fairy can help now. If only he wasn't the fairies' public enemy number one. . .