I recently finished reading The Looming Tower by Lawrence Wright. It is an excellent history of al Qaeda, focusing on the events leading up to 9/11. It also includes a parallel discussion of the FBI, CIA, and NSA investigation of al Qaeda before 9/11.
In particular, I found 4 things Wright reveals about al Qaeda particularly interesting. First, the book begins not with bin Laden, but with an Islamic author, Qtub, who became an ideological leader of Egyptian Islamists who wanted to reclaim and purify their country for their brand of Islam. Second, the Afghan war with the Soviets attracted both bin Laden and the Egyptian Islamists to fight a more global war to protect what they viewed as Islamic territory. Third, it was bin Laden's vision of an extra-national militia which could be easily mobilized to defend Islam globally that would later morph into al Qaeda, a loose coalition of terror groups and cells with the primary goal of expanding Islam globally against the encroaching West, of which the US was seen as the leader. Finally, I was intrigued to learn of repeated offers of help to al Qaeda by Iraq, which saw a common cause against the West.
The history of FBI, CIA, and NSA monitoring of al Qaeda was also interesting. Particularly, their lack of cooperation because of the "Wall" and different information priorities. For example, Wright explains that the CIA was often reticent to share intel with the FBI because the CIA views secrets as currency, whereas widely known info is relatively useless, and the FBI's purpose is primarily to prosecute criminals based on info, which makes it public.
In closing, I highly recommend this book for those who want to know more about al Qaeda and why they seek to destroy us.