The City & The City - Mieville

This is one of the 2010 Hugo nominees.

'The City & the City' starts with the dead body of a young woman. Inspector Borlu of the (fictional) city of Beszel has been called in to investigate. The problem is that he suspects that the victim isn't from Beszel but from the city of Ul Qoma (which is equally fictional). Which turns out to be a big problem.
You see Beszel and Ul Qoma are, well, not really neighboring cities. More like two cities on top of one another. They share the same geographic space but are utterly different. Residents of each are taught from a very young age to 'unsee' and 'unhear' anything and everything from the other. There are serious penalties for violations, including being taken by the mysterious members of Breach.
Borlu suspects that the young woman was killed in one city and dumped in the other, which makes it a matter more serious than simple murder. He must somehow find a way to investigate in the foreign and forbidden regions. But his way is blocked by powerful officials and the whole thing seems like a coverup for something big.

This is a very finely written book. An absolute pleasure to read. It combines the best of hard-boiled detective noir with a convincing eastern European location. The story slowly unravels its most interesting question: what is the nature of the two cities? But there is where the problem lies as well. If there is some mystical, fantastical reason for the Split then I could believe in this strange 'unseeing' existence from residents of both sides. But if it's merely convention that blinds and deafens them then the whole story falls apart. If it's the latter option then it loses any claim to being fantasy. All that's left is an unconvincing and unrealistic situation where whole populations act contrary to human nature. Which is kind of a big problem.
I liked it. Mostly it was great. In the end, I don't think that it quite satisfies the (incredibly loose) genre requirements and I couldn't vote for it to win the Hugo.