The City & The City
More compelling as a concept than it is as a mystery. Though the central conceit is rather confusing at first, once you work out the logistics (and get used to Mieville's thesaurus-assisted writing style -- contumely, ossified, topolganger), it's really a lot of fun just to inhabit his world and consider the metaphysical questions he poses about how much of a role observation plays in shaping reality.
