Artemis Fowl series

When a twelve-year-old evil genius tries to restore his family fortune by capturing a fairy and demanding a ransom in gold, the fairies fight back with magic, technology, and a particularly nasty troll.

Artemis Fowl is going straight-as soon as he pulls off the most brilliant criminal feat of his career . . . but his plan goes awry, leaving his loyal bodyguard, Butler, mortally injured. Artemis's only hope of saving his friend is to employ fairy magic; so once again he must contact his old rival, Captain Holly Short of the LEPrecon fairy police. It is going to take a miracle to save Butler, and Artemis's luck may have just run out. . .

When demons begin appearing on Earth unpredictably, foreshadowing a cataclysmic breakdown of their magic, Artemis and his friends face a new foe--a twelve-year-old girl whose intellect just might match Artemis's own--as they try to prevent catastrophe.