Perdido Street Station / China Miéville.

Enhanced descriptions from Syndetics:

WINNER OF THE AUGUST DERLETH AND ARTHUR C. CLARKE AWARDS * A masterpiece brimming with scientific splendor, magical intrigue, and fierce characters, from the author who "has reshaped modern fantasy" ( The Washington Post )

"[China Miéville's] fantasy novels, including a trilogy set in and around the magical city-state of New Crobuzon, have the refreshing effect of making Middle-earth seem plodding and flat."-- The New York Times

The metropolis of New Crobuzon sprawls at the center of the world. Humans and mutants and arcane races brood in the gloom beneath its chimneys, where the river is sluggish with unnatural effluent and foundries pound into the night. For a thousand years, the Parliament and its brutal militias have ruled over a vast economy of workers and artists, spies and soldiers, magicians, crooks, and junkies.

Now a stranger has arrived, with a pocketful of gold and an impossible demand. And something unthinkable is released.

The city is gripped by an alien terror. The fate of millions lies with a clutch of renegades. A reckoning is due at the city's heart, in the vast edifice of brick and wood and steel under the vaults of Perdido Street Station.

It is too late to escape.