From left-wing 9/11 conspiracy theorists to right-wing Obama-hating "birthers"a sobering, eyewitness look at how America's marketplace of ideas is fracturing into a multitude of tiny, radicalized boutiqueseach peddling its own brand of paranoiaThroughout most of our nation's history, the United States has been bound together by a shared worldview. But the 9/11 terrorist attacks opened a rift in the collective national psyche: Increasingly, Americans are abandoning reality and retreating to Internet-based fantasy worlds conjured into existence out of our own fears and prejudices. The most disturbing symptom of this trend is the 9/11 Truth movement, whose members believe that Bush administration officials engineered the September 11, 2001, terrorist attacks as a pretext to launch wars in Afghanistan and Iraq. But these "Truthers" are merely one segment of a vast conspiracist subculture that includes many other groups: anti-Obama extremists who believe their president is actually a foreign-born Manchurian Candidate seeking to destroy the United States from within; radical alternative-medicine advocates who claim that vaccine makers and mainstream doctors are conspiring to kill large swathes of humanity; financial neo-populists who have adapted the angry message of their nineteenth-century forebears to the age of Twitter; Holocaust deniers; fluoride phobics; obsessive Islamophobes; and more.For two years journalist Jonathan Kay immersed himself in this dark subculture, attending conventions of conspiracy theorists, surfing their discussion boards, reading their websites, joining their Facebook groups, and interviewing them in their homes and offices. He discovered that while many of their theories may seem harmlessly bizarre, their proliferation has done real damage to the sense of shared reality that we rely on as a society. Kay also offers concrete steps that intelligent, culturally engaged Americans can take to reject conspiracism and help regain control of the intellectual landscape.