The author deals with a number of well-known conspiracy theories, from the Protocols of the Elders of Zion to the death of Dr David Kelly, by way of the attack on Pearl Harbor, the death of Marilyn Monroe, the assassinations of JFK and RFK, the moon landings, the Da Vinci code/bloodline of Jesus stuff, and 9/11 (among others) and to my mind demolishes them pretty thoroughly.
But he isn't just interested in debunking. He also examines why people believe in conspiracy theories and why they can exert such a strong grip on them. He points out that conspiracists tend to be on the "losing side" (politically, socially, or economically) of society, and that believing in conspiracies is therapeutic for them. They can explain why they are on the losing side ("we were robbed, deceived") salve their hurt ("the people who deceived us are so powerful, so evil, it's understandable that they appear to be the winners") and then restore their egos ("we have seen the truth, we are so much cleverer than ordinary people who are happy to be sheep-like in their acceptance of things; we are illuminated, in the know, we are special").
Interestingly he is able to develop this line in the light of some recent psychological and biological research which indicates we are genetically hard wired to look for causes and effects. This seems to be related to our developing tool-using capabilities; in order to develop and employ tools we need to think in cause and effect terms. (And of course while some animals to make occasional and specific use of natural objects as tools, humans are the only ones to do so extensively and develop the range of tools to use.) So we are uncomfortable with randomness - if something happens it must because someone caused it to, there's no such thing as an accident, someone must be to blame.
And the more prominent a person is, the more in the public eye, the greater the forces we feel must be needed to pull them down or kill them. Accidents and lone gunmen are for ordinary people, not special ones, and they certainly don't commit suicide.
Not only that but we have a fear of insignificance, of being ignored. If we feel we are being reduced to mere ciphers in a complex society believe in conspiracies is an effective therapy for us.
The author also takes a firm swing at the sort of relativism that exists in some circles and seems fashionable in certain academic circles, that says that one person's perception of what happened is as valid as another's, and that to insist on examining facts and evidence is not helpful.
An engrossing and informative read.