John Harte wrote four previously published books. His account of the Second World War was published in New York in 2017 with the title of How Churchill Saved Civilization. His description of the First World War was published in 2018 as Churchill the Young Warrior. In The Greatest Spy he describes the not-so-well-known situation of a century of world crises from 1896 to the collapse of Soviet Russia in 1991.

The author's postwar experience included undercover work as an investigative journalist on political extremism. In 1948 he uncovered a Fascist plot to take over Great Britain, and spent a year investigating different splinter groups in major cities in the British Isles. He compiled a dossier on their activities which he managed to have presented in the House of Commons by a Member of Parliament--simultaneously providing a scoop for The Sunday Pictorial and Picture Post. His public exposure of Sir Oswald Mosley's postwar British Union of Fascists resulted in the end of Mosley's political career.

He also observed the political situation in Tito's Communist Yugoslavia, and in South Africa during apartheid where he spent ten years working in his main capacity as a top business executive, while continuing his freelance career as a journalist. As a result of his close contacts with the shadowy world of espionage, he became increasingly familiar with the personal characteristics and special skills required by successful spies like the hero of The Greatest Spy.