David Aaronovitch is on a hiding to nothing with a book like this, which seeks to explain how conspiracy theories take root. Or as the sub-title rather teasingly hints - how they in fact "shaped modern history".
Part rebuttal of all manner of loony theories, part sociological analysis of how and why certain tales capture widespread imagination when they do. He manages to pull it off because he's a witty and lively writer, but the proximity to such dense and turgid source material make it tougher going than it should be.
The essence of it is summed up when, exhausted, he pauses in the chapter Holy Blood, Holy Grail, Holy Shit, and asks: "Why do we read bad history books that [are badly written as well as being junk] and not buy in anything like the same numbers history books that are often far better written and are far more likely to give us an understanding of who we are and where we came from."