
The story starts out as a mystery novel in which Detective Borlu has to find the killer of a young murdered woman. The "fantasy" element here is that it takes place in the city of Beszel which takes up the same physical space as the city of Ul Qoma. They are two completely separate cities but they occupy much of the same space. While in one city the citizens cannot acknowledge anything they see or hear that belongs to the other city. To do so would be a "breach" and is considered a very serious crime. The cities are so different that the citizens are able to learn to "unsee" those elements around them that aren't a part of their respective city. They are also afraid of the mysterious people of the Breach that will show up to apprehend anyone that commits a "breach". So what is it that actually separates the two cities? I can't really say without giving away spoilers.
The book works on several levels... first the detective story is pretty solid. As a fantasy novel it is different from anything else I've read. And the whole metaphor of citizens "unseeing" what is around them that isn't a part of "their" city was thought provoking (especially as someone living in Los Angeles, where different sociological/economic groups live in the same space but pretty much in different worlds). I'll give this one an A. (Also, it mentions Harry Potter strangely enough.)