Stormchaser - Paul Stewart

Since his childhood in the Deepwoods, young Twig has always longed to soar above the forest canopy and explore the sky. Now a lowly crew-member on his father's sky pirate ship, the Stormchaser, his dream seems fulfilled. But a much higher destiny awaits Twig as plots and treacheries from many years ago bring the lofty city of Sanctaphrax - home of the Edgeworld's most learned academics - to the point of disaster.

For the city is built on a giant rock, floating high in the sky, its future balance dependent on stormphrax - a valuable yet extremely volatile substance obtained only from the heart of a Great Storm at the very instant it unleashes its most intense power. And only a ship such as the Stormchaser could risk entering the Storm...