The City & the City — China Mieville

When a murdered woman is found in the city of Beszel, somewhere at the edge of Europe, it looks to be a routine case for Inspector Tyador Borlil of the Extreme Crime Squad. To investigate, Borhi must travel from the decaying Besiel to its equal, rival, and intimate neighbor. the vibrant city of Uf Qoma. But this is a border crossing like no other, a journey as psychic as it is physical, a seeing of the unseen. With Ul Qoman detective Qussim Dhatt, Borin is enmeshed in a sordid underworld of nationalists intent on destroying their neighboring city and unificationists who dream of dissolving the two into one. As the detectives uncover the dead woman's secrets, they begin to suspect a truth that could cost them more than their lives. What stands against them are murderous powers in Besiel and in Ul Qoma -- and, most terrifying of all, that which lies between these two cities.