Edge Chronicles: Stormchaser (The Edge Chronicles)

Since his childhood in the DeepWoods, young Twig has always longed to soar above the forest canopy and explore the sky. Now a crew member on his father's sky pirate ship, the Stormchaser, his dream seems fulfilled. But a much higher destiny awaits Twig. The lofty city of Sanctaphrax--built on a giant rock, floating high in the sky--is at the point of disaster. The city's future is dependent on stormphrax--a valuable substance available only from the heart of a Great Storm. And only the Stormchaser, with Twig onboard, could risk entering a storm. . . .From the Hardcover edition.