Artemis Fowl #3: The Eternity Code book by Eoin Colfer

Artemis Fowl is going straight. As soon as he pulls off the most brilliant criminal feat of his career.

At least, that’s the plan when he attempts to sell his C Cube, a supercomputer built from stolen fairy technology. When his efforts to broker a deal for the Cube with a powerful businessman go terribly wrong, his loyal bodyguard and friend Butler is mortally injured. The only thing that will save him is fairy magic, so once again he must contact his old rival, Holly Short.
It’s going to take a miracle to save Butler, and Artemis’s luck may have just run out.