I have just finished reading David Aaronovitch's brilliant demolition of conspiracy theorists, Voodoo Histories, and it is one of the most entertaining books I've read in ages. Aaronovitch shows us how the Protocols of Zion were a poor copy of a mid-19th century French novel; reminds us of western gullibility over the Stalin show trials; and moves contemporaneously to bien pensant beliefs in the 'murder' of Hilda Murrell and David Kelly (his demolition of the sanctimonious Lib Dem MP, Norman Baker is particularly powerful). Aaronovitch shows how those who think Roosevelt sat back while Pearl Harbour was bombed are as likely to think that 9/11 was a huge CIA plot. And he shows the absurdity of the beliefs in conspiracies around the deaths of JFK (I hadn't known that Oswald had tried assassination previously), Princess Di and Marilyn Monroe.
Conspiracy theorists tend to produce books full of apparently credible references, but the sources tend either to be other conspiracy theorists or chance media reports that have later been updated. Many theories are harmless and wacky; but beliefs in conspiracies can have hugely harmful effects, as with the Protocols (that fed Hitler's anti-semitism) and the acceptance of the Stalinist show trials by Western Communists or McCarthyism. As Aaronovitch says, the book is a brilliant antidote to the pub or dinner party bore who claims to have 'irrefutable proof' of a conspiracy. Thanks to his work, it is possible to retort credibly that 'stuff happens'. Do read it.