Inspector Tyador Borlú, of the Extreme Crime Squad in the European city-state of Besźel, investigates the murder of Mahalia Geary, a foreign student found dead with her face disfigured in a Besźel street. He soon learns that Geary had been involved in the political and cultural turmoil involving Besźel and its twin city of Ul Qoma. His investigations start in his home city of Besźel, lead him to Ul Qoma to assist the Ul Qoman police in their work, and eventually result in an examination of the legend of Orciny, a rumoured third city existing in the spaces between Besźel and Ul Qoma.This is actually the first Mieville book that I have been able to make it through. He's a huge award winner in the SFF world and has even done some writing for roleplaying games. I think the reason that I was able to enjoy the book is that it is a science fiction novel wrapped in a crime procedural.
The concept of two cities existing side by side with innumerable interstices was a difficult one to be drawn in to. He is a skillful writer though and as you push on through the book he makes the idea plausible.
I would definitely recommend this book as a place to start if you want to dip your toe into the works of China Miéville.