Artemis Fowl and the Opal Deception by Colfer Eoin

Evil pixie, Opal Koboi, is back and she's more dangerous than ever. This time she doesn't just want power over the fairy People – this time she wants the humans too. Captain Holly Short is the only fairy with a hope of stopping her, but as Holly knows, it takes one genius criminal mastermind to fight another. But the 14-year-old genius that Holly is thinking of doesn't even remember that fairies exist. How is she going to convince Artemis Fowl to help her stop Opal? Gold usually does the trick, and this time is should be no different. Or is Artemis changing?