Ask Richard Gage how he came to become obsessed with what he calls the “truth” about 9/11, and you hear what sounds an awful lot like a story of religious conversion. It was March 2006, and the mild-mannered California architect was driving down the Pacific Coast Highway on his way to a construction meeting. Bored, he flipped on KPFA 94.1 FM, a listener-supported “free-speech” station out of Berkeley — “to hear what the communists were talking about,” as he later told me in a 2009 interview. Up to that point in his life, Gage had been a staunch “Ronald Reagan...