Glenn Simpson, the former Wall Street Journal reporter turned high-priced “oppo” merchant, didn’t like to think of himself as a private investigator. He preferred to describe what he and his firm, Fusion-GPS, did as “journalism for rent,” an activity a class above spying, because a journalist can’t just say what he or she thinks.

“You have to prove it,” Simpson said. “And that imposes a discipline to the investigative process that people in other fields don’t really absorb… When you’re a spy, you really don’t have to get into a lot of that stuff.”

Spooked, the meticulous new book on private spying by former New York Times reporter Barry Meier, reads like a direct rebuttal to Simpson, the book’s central character. “There is little question that private investigators take on legitimate assignments,” writes Meier at one point. “Still, everyone in the industry knows its secret — that the big money is made not by exposing the truth but by papering it over.”

Meier, a two-ti…