The City & The City, by China Mieville
Mieville can do no wrong, as far as I’m concerned. He’s stepped away from the completely fictional universes (New Crobuzon, anyone?) of previous novels while still maintaining a fantastical city. Two cities in the same location are separated by nothing more than the idea of separation, and one man works to explore and then uphold the boundaries between. A lovely mindtwisting novel that leaves you wondering how an author could conceive of, much less portray, such a concept.