To Milton William Cooper, the truth was out there. Way out there. Not in an “X-Files” way — in fact, after dabbling in extraterrestrial theories, Cooper eventually thought U.F.O.s were “possibly the single greatest hoax in history,” just another threat fabricated to keep a docile public in thrall to an increasingly one-world government. Cooper was in the earthly conspiracy business. He was killed in a shootout with police shortly after the events of Sept. 11, 2001, which many say he prophesied. His book “Behold a Pale Horse” has been wildly popular among conspiracy theorists since its publication in 1991. In “Pale Horse Rider,” Mark Jacobson tells the story of Cooper’s life, and explores the roots and consequences of his distinctly American brand of fevered conjecture. Below, Mr. Jacobson discusses Cooper’s appeal to prisoners, the current crisis of faith in America and more.

When did you first get the idea to write this book?

It’s complicated. I first became aware of Cooper more than 20 years ago because of some of the crazier things he said, especially about the Kennedy assassination. After nearly 30 years of increasingly byzantine theories of who killed the president, his idea was that the driver just turned around and shot him. Which was cracked, but there was something interesting about it.

Another time, I was walking down my street and saw Ol’ Dirty Bastard, and he was reading “Behold a Pale Horse.” Several rappers had quoted this book. I knew that Cooper was some big white guy who lived on a hill in Arizona. This struck me as one of those journalistic conundrums; something you take away for later reference.

Then in 2014, when reaction to the Obama presidency crossed over from National Enquirer land to mainstream, I was reading the kind of thing on the internet that I hadn’t seen before. There was a fervor behind these comments that seemed so out there to me. Then I began to realize: This isn’t a joke to them; they really believe it. I started to think, what about Bill Cooper? And then it became kind of obvious that he was this lost key — to me, anyhow — about what was happening.