It turns out that 9/11 conspiracy theories look a lot less plausible when subjected to fact checking. The looniest of the theories never really made sense, but given the convenience of 9/11 in enabling the massive power grab by the Bush administration, and its use as a pretext for the long-desired invasion of Iraq (long desired by Cheney and the neo-cons, that is), there was some appeal to the idea that the Bush administration possibly had prior knowledge of the coming attacks. Even that is a bit of a stretch: it would imply better foresight and planning than was ever evident in that administration.
This book pretty effectively takes apart each of the claims of 9/11 conspiracists, showing how the theories are often based on made up facts, facts taken out of context, and pure conjecture. One common theme in 9/11 conspiracy theories is that if the ‘official’ explanation of an event is not absolutely airtight, then we should conclude that the only alternative is a massive conspiracy involving hundreds or thousands of federal employees. A sort of ‘god of the gaps’ argument, in other words.
The authors of this book seem a bit mystified that so many people have expressed belief, or at least have not entirely rejected, the conspiracy theories. I have my own theory about that: when your government routinely lies to you, keeps its decision making secret, tells you that the decisions it makes are based on information that you don’t have (and you later discover that that, too, was a lie), and does its utmost to punish those who seek to publicize the truth - given all that, you are likely to suspect that anything the government tells you is a lie. If Cheney had said the sun rises in the east, I would have been inclined to get up at dawn just to verify for myself.