I have to say that I strongly disagree with the rather indifferent reviews of this book posted so far. I have read all of Reynolds' books to date and this makes a strong claim to be his best.
For starters, and almost incidentally, it is the best steampunk novel I have read. Reynolds produces a plausible plot device for examining a society trapped at a particular technological point, and his steam or dieselpunk technology is grittily plausible and realistic, not a series of fashion accessories or nostalgic anachronisms, as is all too common in this genre.
Secondly, this book requires a bit of intellectual effort on the part of the reader. The reader is required to use some imagination and to draw inferences and make conclusions from tiny nuggets of fact dropped into the characters' conversations. The book contains no "infodumps". The true nature of Spearpoint is not spelled out directly, even at the end of the novel. An observant reader will fairly quickly come to a huge revelation about the nature of Spearpoint's world which never becomes remotely obvious to any of the characters involved. One particularly ironic point is the existence of a quasi-religious "Testament", which most of the characters dismiss as mythological, but the more objective viewpoint of the reader can see is largely historical fact about the planet's history.
There are also some excellent action scenes, particularly a desperate airship assault on the city in the face of progressive technological failures, reducing the crew from machine guns and diesel engines to cutlasses and crossbows in the space of ten minutes. The characters are excellent, particularly a foul mouthed bodyguard heroine.
If you like your SF one-dimensional and spoon fed to you as easily digestible gloop, this book probably isn't for you. If you are willing to use your intellect and your imagination to fill in the tantalising gaps left by the author you will be amply rewarded.