Warning: mild spoilers above the fold, big spoilers below. There is no way to describe this book without spoilers.
The protagonist is a detective solving a mysterious murder. A body has turned up in the fictional Eastern European city of Beszel. The problem: the body has been dumped across an international border; the victim lived in, and was almost certainly murdered in, the neighboring fictional Middle Eastern city of Ul Qoma.
These aren't like East and West Berlin, or Jewish and Arab Jerusalem, sharing a single contiguous unambiguous border. The cities occupy the same physical grid of streets with borders and "shared" areas crisscrossing the literal topographical ("grosstopic") area. Only some unfathomed and possibly unfathomable force prevents the citizens of each city from perceiving and interacting with each other. It's not just that it wasn't legal to dump a body across the border - it shouldn't have been possible at all.
I cannot tell you what makes The City & the City, by China Mieville, so good without spoiling the whole thing, but I will tell you that it does not betray the trust of a reader who expects mysteries to be about something. This is not Lost. There really is a secret to the Cities, it makes sense, and it is big enough to justify the story. To the right kind of reader, this is recommendation enough - if so, go and read it.
The big spoilers are below the fold.