In 1999, Tom O’Neill was commissioned by Hollywood-centric film magazine Premiere to investigate exactly what had — by then — happened a mere 30 years earlier, when a small-time criminal, Charles Manson, was unveiled as the mastermind behind one of the most brutal and defining crimes of the 20th century. It must have seemed like an easy job back then, but by the time the title ceased trading in 2007 O’Neill still didn’t have a piece for them — as he explains with great honesty in his self-deprecating and far-ranging 2019 book Chaos: Charles Manson, the CIA, and the Secret History of the Sixties.

That book took all his research — exhaustive and authoritative but frustratingly inconclusive — and threw it all up in the air in exasperation, which is not something many writers are inclined to do. It’s also not something that documentary legend Errol Morris is wont to do either, so it’s interesting to...