Meet Artemis Fowl: criminal mastermind, wildly wealthy scion, 12-year-old. His plot? Trap a fairy and hold it ransom for gold. And in the beginning, as he pokes his way through a subterranean world of goblins, elves, and assorted magical beasties, things bubble along happily. It’s in the book’s final section — devoted entirely to a Die Hard-style stand-off at Fowl Manor — that reverse alchemy kicks in and things turn leaden. (There’s so much high-tech babble and raw exposition on the page that you can practically hear Miramax cackling: It’s Spy Kids for the D&D set!) The Harry Potter comparisons are inevitable, but J.K. Rowling’s work is so singular that the juxtaposition demonstrates only the difference between a great children’s book and a simply good one.