Book The Edge Chronicles 6 Books Collection — Paul Stewart

Fourteen-year-old Quint Verginix is the only remaining son of famous sky-pirate Wind Jackal. He and his father have journeyed to the city of Sanctaphrax – a great floating rock, bound to the ground below by a chain, its inhabitants living with their heads literally in the clouds.
But the city hides a dangerous secret: deep inside the great rock, something horrible lurks. With his father away, Quint may be the only one who can save Sanctaphrax from the dreaded curse of the gloamglozer . . .