Charles Duff (1894-1966) was a British author of books on language learning and other subjects.
Duff served as an officer in the British Merchant Navy in World War I and then in the intelligence division of the Foreign Office and Diplomatic Service. He resigned from the Foreign Office in the 1930s when he realised it was solidly supportive of fascism in Spain and ready to back a similar system in Britain. I couldn't paint golden angels - Chapter II
After he retired, he taught linguistics and languages in London and Singapore while writing travel guides, histories, satires, and a series of text books for the active self-learner