The best book on the people around (and perhaps behind) 9/11: "Another 19" by Kevin Ryan. http://www.another19.com
The "other 19" is group of insiders who have been running together for decades, some of them going as far back as the Nixon administration.
They're specialists in things like "continuity of government", coups, advanced demolition techniques, arms smuggling, covert military operations, news media manipulation, money laundering, covert dealings with Middle Eastern terrorists, security systems for the World Trade Center, and air defense systems.
They're proven killers, liars, and thieves who have demonstrated they'll do just about anything to get what they want.
As a group, these 19 actually ran US air defense system AND security for the World Trade Center on September 11, 2001. They also managed the clean up and the (laughable) investigation, set post-attack military plans, and explained it all - including the "need" to invade Iraq to the press.
They were involved in things like the Phoenix Program, drug running in Vietnam, Iran Contra, the First Gulf War...
All in all a very experienced group - with close ties to the Bush family and its cronies - that was more than capable of pulling 9/11 off.