The Looming Tower, by Lawrence Wright. Summary: this gripping book is a look at the history and development of al-Qaeda leading up to 9/11. Very highly recommended.
The subject of this book is easy to discern, but amazingly unknown in the broader US population. The writing is excellent, and Wright gives a deep and sympathetic account of al-Qaeda's underpinnings without losing his essentially American perspective. Ever heard of Sayyid Qutb? If not, read this book. Nearly every page contained a revelation of some sort for me.
Wright also goes into the infuriating story of the few intelligence officials who had some inkling of 9/11 and failed to prevent it, for mainly idiotic reasons. I won't go into detail save that the CIA emerges as the main villain.
Really, I can't recommend this book highly enough. It should be required reading for every American. Enough said.