The Edge Chronicles 2: The Winter Knights: Second Book of Quint

In the great floating city of Sanctaphrax, blizzards howl through the streets as the Edgeworld descends into an endless winter. Quint, the son of a sky pirate, has just begun his training at the Knights Academy - training that involves heading out over the Edge on tethers to develop his flying skills. But when Quint breaks the rules and head out to Open Sky on his own, he runs into the great sky leviathan known as a cloud-eater, and must use all his skill and ingenuity if catastrophe is not to strike the Edgeworld...