Reynolds develops the idea of a dying world in the far future where the laws of physics vary from place to place, based on the changing resolution of the underlying “grid” on which matter is laid out. In the high-resolution areas, extremely precise things like nanotechnology work; in others, cybernetics are fine but nanotech is out; in some, electronics works but microchips don’t (I did a double take early on in the book when someone has a rotary dial cellphone); in some, only steam technology works (and there is a highly amusing encounter with a cyborg warrior who has been retrofitted to survive in a steam-only zone); in the lowest habitable regions, it’s just living beings made of flexible proteins. (Some areas don’t support life at all.) Changing zones, however, plays hell with the nervous systems of humans, leading to a need for medicines to help them adapt when they travel.
The center of the action is a Big Dumb Object named Spearpoint: an artificial mountain spiraling up into the stratosphere, on which the zones of varying technology vary quite quickly, with nanotech-enabled “angels” in the Celestial Levels soaring over places with names like Circuit City, Neon Heights, Steamville, and Horsetown. Our hero, Quillon, is a fallen “angel”, using his expertise in medicine to hide out in Neon Heights working as a coroner... until he discovers that his former colleagues from the Celestial Levels intend to hunt him down.
This leads to an odyssey into the outside world, complete with some never-well-explained antagonists, the Skullboys, who provide a Mad Max-ish postapocalyptic vibe to the tale, discoveries about the forgotten history of the world, and an eventual return to Spearpoint. The main threads of the story get wrapped up at the end, but Reynolds doesn’t give a lot of insight into the motivations of the antagonists, and while he hints of conspiracies going on, he never sheds much light on them. (It’s realistic that the protagonists never get a chance to find out, but dissatisfying for the reader.) A fun read, but not up to the quality of his other work.