The rook trilogy - Paul Stewart

The Last of the Sky Pirates
Rook Barkwater lives beneath Undertown, the bustling city of the Edgeworld.He dreams of becoming a librarian knight and sets out on a perilous journey. In the heart of the Deepwoods, Rook encounters a mysterious character - the last sky pirate . . .
Vox
High in the crumbling Palace of Statues, little more than a prisoner of the all-powerful Guardians of the Night, Vox Verlix plots and schemes . . . Rook Barkwater finds himself embroiled in Vox's scheming.He must stop him or the Edgeworld could fall into total chaos . . .
Freeglader
Undertown is destroyed! Gnokgoblins, waifs and cloddertrogs huddle by the ruins, preparing for a mass exodus to the Free Glades. Rook Barkwater knows the journey will be full of peril. Dangers are everywhere, and in the Goblin Nations, the tribes are massing for war. Can Rook and his friends preserve freedom in the Edgeworld?