It’s hard to explain Tom O’Neill’s new book Chaos: Charles Manson, the CIA and the Secret History of the Sixties without sounding like a conspiracy theorist down a rabbit hole — you try telling your friends that a reporter spent two decades researching the links between one of America’s most notorious criminals and the government’s super-secretive mind-control program Mkultra without getting a few snickers.

Of course, imagine being the journalist writing it, and you find yourself in an even more uncomfortable position; that’s exactly why...