Flash Boys describes what high frequency trading (HFT) is and why it's done. In spite of the very technical nature, the books manages to make this pretty understandable.

The book also chronicles the story of a band of people that try to prevent the flashtraders from messing with your and mine stock orders.

Everything is described from an outsiders angle and gradually peels away the many different layers of the story. A technique which I really liked for this subject.

Overall, I learned a lot and would suggest this to any one interested in stocks and trading. It will probably not help you in making better decisions, but it's great background information.