Terminal World by Alastair Reynolds

Alastair Reynolds revolutionized the science fiction genre with his critically-acclaimed Revelation Space novels. Now, the award-winning author presents a future of technological wonders-from every era.
In a far-distant future, Spearpoint, the last human city, is a vast, atmosphere-piercing spire. Clinging to its skin are the zones: semiautonomous city-states, each of which enjoys a different-and rigidly enforced-level of technology.

Following a botched infiltration mission, enforcement agent Quillon has been living incognito, working as a pathologist in a morgue. But when a near-dead angel drops onto his dissection table, his world is wrenched apart.

For the angel is a winged posthuman from Spearpoint's Celestial Levels. And with the dying body comes bad news: Quillon must leave his home and travel into the cold and hostile lands beyond Spearpoint's base. But he can neither imagine how far the journey will take him-nor comprehend how much is at stake.