Two cities occupy the same physical geography. Same streets, same buildings, same air. But the citizens of each have been trained from childhood to "unsee" the other. Its people, its cars, its architecture. There's no wall between them. No border patrol, no force field, no magic. Just an enforced act of collective blindness, maintained by surveillance and consequence. To acknowledge the wrong city, even accidentally, is the gravest crime in both.
The book is structured as a murder mystery, but the investigation is really a vehicle for the central question. How much of what divides places and people is just an agreement to look away?
Winner of the Hugo, Arthur C. Clarke, World Fantasy, and BSFA Awards. 320 pages. Goodreads 3.91 with 130k+ ratings.