The first one that pops into my mind is China Miéville's The City and the City. The idea of two cities coexisting side-by-side the way Miéville presents them is a mind bender. Two cities that are culturally completely different, including their judicial/legal status and how they are governed, yet they share the same topographical space. A particular street may belong to one city and an adjacent street to the other. Alternatively, a street/area may start out belonging to one and at some point switch to belonging to the other. The inhabitants of either city fastidiously try to 'unsee' the other for fear of committing 'breach', a surreal form of felony. Getting around to the idea of 'unseeing' took some effort in this hardboiled detective tale. And while the protagonist is an official detective, there is still a definite noir ambiance around the whole story. Before reading TCatC, I would have nominated Miéville's Perdido Street Station, which is full of weird goodness. But I think TCatC tops it in terms of uniqueness precisely for being set in our own reality (albeit in two fictional, eastern European cities), which makes this surreal rift between the closely linked cities even more pronounced than if it were set in an alternate fantasy world.