Pincher is best known as the author of the book "Their Trade is Treachery" (1981), in which he publicized for the first time the suspicions that MI5's former Director General Roger Hollis had been a spy for the Soviet Union, and described MI5's and MI6's internal inquiries into the matter. Pincher was at one point close to Peter Wright, who, he knew, suspected Harold Wilson of having been a Soviet agent, and according to the biography of Wilson written by Ben Pimlott, Pincher was trying to get information from Wright so that he could accuse Wilson in a public setting in the near future.