Artemis Fowl Book 5: The Lost Colony (by Eoin Colfer)

But someone else, another genius child named Minerva Paradizo, has figured it out, too, and manages to capture a small imp named No. 1. Artemis agrees to help Holly and the Fairies rescue the imp before more humans find out about him and start searching for more -- which might lead them to the Fairy underground.

As the series evolves, Artemis has lost the last traces of his criminal bent, almost becoming the millennial version of a boy scout. He and the Fairies are now solidly on the same side and good friends. Even the violence has been dialed back a bit. With his brilliance, technology, family organization, and world-spanning adventuring, Artemis has become a sort of Tom Swift for the 21st century.

Five books into the series the relationships and motivations are getting more complex, so it is best to start with the first book. Author Eoin Colfer seems to like putting Artemis up against other geniuses but Minerva, who doesn't really mean any harm, is no Opal Koboi (for the uninitiated, she's the maniacal villain from books 2 and 4). So with THE LOST COLONY the series returns to the pleasure of seeing Artemis, always in charge and unflappable, work out his complicated plans; he's not off-balance and one step behind, like in the fourth book.