Warning: mild spoilers above the fold, big spoilers below. There is no way to describe this book without spoilers.
The protagonist is a detective solving a mysterious murder. A body has turned up in the fictional Eastern European city of Beszel. The problem: the body has been dumped across an international border; the victim lived in, and was almost certainly murdered in, the neighboring fictional Middle Eastern city of Ul Qoma.
These aren't like East and West Berlin, or Jewish and Arab Jerusalem, sharing a single contiguous unambiguous border. The cities occupy the same physical grid of streets with borders and "shared" areas crisscrossing the literal topographical ("grosstopic") area. Only some unfathomed and possibly unfathomable force prevents the citizens of each city from perceiving and interacting with each other. It's not just that it wasn't legal to dump a body across the border - it shouldn't have been possible at all.
I cannot tell you what makes The City & the City, by China Mieville, so good without spoiling the whole thing, but I will tell you that it does not betray the trust of a reader who expects mysteries to be about something. This is not Lost. There really is a secret to the Cities, it makes sense, and it is big enough to justify the story. To the right kind of reader, this is recommendation enough - if so, go and read it.
The big spoilers are below the fold.
The plot, while straightforward, is a red herring; the true mystery is the setting: the two neighboring cities of Beszel and Ul Qoma. At the book's beginning, the reader is given the strong impression that what separates the cities is some either supernatural or technological force; that it is actually impossible for someone in Besz to see across the street, if the other side of the street is in Ul Qoma. Gradually, we learn that the locals practice something called "unseeing," and the barrier is not passive, but something actively enforced by a secretive organization called Breach, named after the type of meta-crime they exist to prevent: looking across the border, or even interacting with someone or something on the other side.
We learn that in "crosshatched" spaces (parts of both Beszel and Ul Qoma), the lack of interaction is not really an effortless state, but the result of careful subconscious avoidance, which is only "easier" than committing breach, through force of habit.
I and others around me unsee people all the time. People on the Metro or the sidewalk fail to acknowledge one another's presence in ways that would be rude in a less crowded space. If I'm looking for someone in a crowded restaurant or bar, or figuring out whom to introduce myself to at a catered party, I automatically skip over the servers. If I'm having a conversation at a social gathering and a stranger walks over, I try to include them in the conversation - but in an elevator, I may not notice them at all.
I unsee spaces, objects, and behaviors, too. As a man, if I want to look more attractive, I think about the gym, or the men's section of the department store, or the barber shop. I don't even notice the department store's makeup counter; it's just an empty space to me. As a not particularly handy man, I have the same attitude towards large parts of home improvement stores. Raised Jewish, I pass a church on Sunday and it doesn't even occur to me that it would be possible for me to enter. For a while after college, it never occurred to me to take a cab when it was possible to get somewhere by bus or train or in my own car. It's not that I decided I couldn't afford it; it's that I never thought about it. For the same reason, I have trouble even considering to rent a car when traveling, because before I turned 25 it was prohibitively expensive, so I mentally categorized it as a thing I don't do.
We learn that Besz and Ul Qoman citizens are trained from childhood to see only their own city. Foreigners have trouble understanding and respecting these rules, and often breach inadvertently. Since this is understandable due to their lack of acculturation, they are only expelled, rather than punished in whatever strange unknown way Breach decides. But no one caught committing Breach is ever seen in either city again.
We learn that Breach is a political institution, subject to Ul Qoman and Besz authority, jointly, despite its mystery.
Even regular nations are obviously social constructs, and Beszel and Ul Qoma even moreso. The protagonist recounts that in past decades, the training for foreigners and border crossers focused on physical characteristics and crude stereotypes; now the focus is on more explicit signifiers like clothing and language. Occasionally, people from one city emigrate to the other, and it's necessary to have signifiers of nationality that work for them too.
Imagine a Besz who needs or wants to do something in Ul Qoma. Maybe they have business to transact. Maybe they have friends over there. Maybe they just want to visit. They can't just go there in Besz costume, unmarked, after they cross the border. After all, without the cultural cues indicating that they are "in" Ul Qoma, nearby Besz will fail to "unsee" them, and nearby Ul Qomans will carefully avoid noticing them. Instead, they must wear special markers to designate them as visitors to Ul Qoma, as temporary Ul Qomans, so that the appropriate seeing and unseeing can occur. They inhabit that space, even though they don't have a permanent residence there. This permits them to interact with people and things "in" Ul Qoma, and of course while there, they're subject to Ul Qoman laws.
Many actual countries don't really have this state of affairs, but maybe you've been someone's guest at a private club, or visited a corporate office building with a "visitor" badge, or sat in on a service in a house of worship whose religion you do not share. Maybe you've dressed in drag.
Then, sometimes people want to move permanently. Maybe they are moving for a job. Maybe they are escaping persecution, political or otherwise, in the other city. Maybe they fell in love with someone from the other side. An Ul Qoman can become Besz just like anyone can change their nationality. They will still look a little different - they might stick out a bit because of their facial structure, or their manner of dress, or their accent - but to say that they are not "really" Besz would be a very aggressive, offensive thing to say.
Someone might even move because they prefer the other city's way of life. Some immigrants to America have said that they always felt American at heart - and I'm sure some immigrants to France have said the same about it. Why couldn't someone born in Ul Qoma grow up feeling more Besz than Ul Qoman? But of course in another sense they're not really Besz until the paperwork is complete.
Visiting and immigration don't really threaten the system. Nor do the existence of Ul Qoman or Besz nationalists, who believe that their city is better and should take over the other. An Ul Qoman might be a little personally uncomfortable around Besz nationalists, but mostly they're not likely to interact much. Political unificationists are a little scarier, because they're for erasing the barriers - but they accept that there really is a barrier that could be erased. But what's really terrifying is people who breach.
Why is breaching so terrible a crime? Because it threatens the integrity of the cities. The only way it's possible to have a law for the Besz and another for the Ul Qomans, the only way it's possible for these two interlocking, crosshatched cities to exist in the same "grosstopic" space, is the lack of interaction. If someone can just walk from a Besz space into a neighboring Ul Qoman one, at will, interact with people, and leave, how are the cities to have their own customs? Laws? Regulations of trade? Distinct national characters? Separate policing would be impossible, because anyone could just cross the state line. It wouldn't mean anything to be Besz or Ul Qoman anymore. The two nations would in effect cease to exist.
So someone who thinks of themselves as truly cosmopolitan - or just doesn't particularly identify as bound to Beszel or Ul Qoma - threatens to shatter the boundaries that make their separate lives tenable. The Breach organization itself is necessary to police these boundaries, to make it not even slightly tempting to trespass, but it is also an uncomfortable reminder of this necessity, and usually just not thought of. Like ancient gods or legends who magically transform from one kind of thing into another, they appear to do the supernatural just by crossing certain streets.
Reading The City & the City gave me a sort of intellectual compassion for those whose "national" identity schema differs from mine in some way. Those of us who live "in" certain spaces shouldn't assume that others, who find their assigned space intolerable, are trying to cheat the system - they're just looking for a way of living that meets their needs. We would do well to try to respect their desire to find the city they belong in, and to travel back and forth if their life requires it. A foreigner visiting Besz or Ul Qoman might find it perplexing to have to remember which city they're in, especially if they're standing in a cross-hatched area. It doesn't seem to them like they really are in one and not the other. Even if their parents are ethnic Ul Qomans, and they speak Ul Qoman, and they entered the city through an Ul Qoman border crossing - why on earth shouldn't they pop across the street to grab a cup of coffee in "Beszel"?
And those of us who are more cosmopolitan in some way would do well to remember that while we're not trying to hurt loyal citizens, every time we Breach we're eroding the very national boundaries that define them. The cities we live in might not be basic natural categories, but they are nonetheless real; they are people's homes. We're not just claiming freedom of movement in open spaces - we're walking right through what they see as walls.