The Lost Colony (Artemis Fowl, book 5) by Eoin Colfer, 2006

Artemis Fowl, 14-year old billionaire Irish genius, has tangled with fairy-folk, reformed, and finally earned their trust, but when he attempts to decipher an unraveling magical space/time paradox that brings demons stranded in another dimension to Earth, he finds that a new terrestrial rival, French millionaire 12-year old Minerva Paradiso, has gotten there first. Diabolically clever and devilishly pretty, Minerva is almost a match for Artemis, though puberty may be his real undoing. This fifth installment takes the Artemis series to newly implausible heights of cosmic-fairy-techno-crimebusting-derring-do, with romance for seasoning and several promising hooks left for #6. Leave logic at the door and ride the roller coaster. Recommended (but reading the prequels in order is vital).