This is a book published in 2011 by Canadian journalist Jonathan Kay and which is a history of contemporary conspiracy movements, with a focus on the 9/11 conspiracies - their theories and their participants. His main theory is that the conspiracy movements in America have been gathering more and more momentum in the last 10 to 15 years, with more people joining them than ever before. He attempts to define the factors that cause people to believe in these conspiracies wholeheartedly and uncritically.
I found this book fascinating. Initially, I had no desire in reading it but then I picked it up and found it really interesting because Kay, in part, spent a good amount of time immersed in "Truther" culture researching the book and interviewing subjects, such as Alex Jones. This book is an alarm about an increasing paranoid that isn't really based in reality at all. What I found fascinating wasn't necessarily the historical aspect, although being provided the history of conspiracy movements was invaluable in providing me context for the current movements. The most fascinating part of the book were the oral histories given to him by some of the participants, the reasons that were given for their attraction to these theories and Kay's analysis as to why such seemingly intelligent people were subjected to such abject paranoia.
Definitely a must read. If you like it, Kay has a blog that gives more information about his book and his theories.