The City and The City, is an amazingly rich and detailed book, when it is describing its two main characters: Besźel and Ul Qoma. The human characters in the book are largely flat and two dimensional. I never got into them or got emotionally attached. The same cannot be said for the cities (Besźel and Ul Qoma).
The novel begins with a crime committed in Besźel and the detective there is eventually lead to Ul Qoma (and eventually Breach, but that's another story).
The most interesting part is that the two cities exist simultaneously in the same place. They aren't magically juxtaposed, but the citizens of each city willfully train their senses to ignore the other city. You can walk down a "cross-hatched" city street and both cities exist, both citizenries exist, but you will only see/hear/smell the one you belong to. If you think about this, you do it every day. You don't see the things you want to, you don't hear the things you want to. This whole world has been created by exaggerating what we, here and now, do already (whether willfully not seeing something unpleasant or just filtering something out because you cannot focus on everything 100% of the time), but to such an extremeness, two entire places are completely created. To react/notice/see (vs "unsee") the other is to commit a crime called "Breach" and the consequences can be permanent.
I loved the world building, I love the physical, political, economic differences between the two cities. One is an up and coming, thriving, modern city, the other is an old, run down, struggling city.
The actual mystery is there, it is solid and seems to be well thought out with appropriate details and information, but the characters are weak, even if everything is reasonable and well done, it just feels like the mystery is the backdrop to showcase the city instead of the other way around, instead of having a main story with a traditional city backdrop.