Interesting and well-written, but ultimately probably better served in simplified, movie format where we would at least get to see pretty visuals. 
 The concepts of two cities co-existing on top of each other, and where it is the utmost crime to peek over to the other city, is a very original idea, but also not convincing. It is impossible to police hundreds of thousands of inhabitants to prevent them from "breaching", especially when it is finally revealed as to what the mysterious Breach entity is and how they operate. 
 Breach ended up being...somewhat underwhelming. And the crime investigation was so full of red herrings that by the time the investigator got explain the crime at the end (in one massive infodump), I stopped believing. I couldn't see where/how he managed to come to those conclusions. Additionally, it seemed that the guilty parties involved in the murder was jumping through way too many hoops and shedding a lot of blood over something that....was really not that serious. If I sit and analyze it, I can see how it all works out....but I prefer to judge a book by how I feel _while_ I read it, and not rationalize it all after the fact.