The City and the City by China Miéville

While this book's plot centers around the murder of a grad student studying archaeology, its fascination comes from its setting--an unidentified place on the edges of Europe where two cities exist in the same landscape. Their borders are zealously guarded by Breach--both a concept ingrained in the population from a very young age, and an extralegal agency that handles individuals who see or sense things outside of their city. I hesitate to say too much about the plot, because I don't want to spoil anything for anyone, but I will say that it was very well-done and kept me guessing till the final pages. Do yourself a favor and read this book as soon as possible.