Charles Duff (1894–1966, Cathal Ó Dubh) was an Irish author of books on language learning and other subjects born in Enniskillen, County Fermanagh.
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, claiming it was solidly supportive of fascism in Spain and ready to back a similar system in Britain.
After he retired, Duff 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. He was fluent in seven languages. His many translations included works by Francisco de Quevedo, Émile Zola, B. Traven, Maxim Gorky, and Arnold Zweig.
Duff's best known book is A Handbook of Hanging. This also covers electrocution, decapitations, gassings, innocent men executed and botched executions. It has been reissued intermittently in the UK, e. g. in 1948, 1953, 1954, 1974, etc., and in the United States in 1999, with an introduction by Christopher Hitchens.
- A Handbook on Hanging (later editions: A New Handbook on Hanging)
- Anthropological Report on a London Suburb (satire), 1935
- Russian For Beginners
- Spanish For Beginners
- French For Beginners
- Italian For Beginners
- German For Beginners
- How to Learn a Language, 1947
- French For Home Study
- England and the English
- The Basis and Essentials of French, 1940 (2nd edition)
- The Basis and Essentials of German (with Richard Freund)
- The Basis and Essentials of Spanish
- The Basis and Essentials of Italian
- The Basis and Essentials of Russian (with Anissime Krougliakoff)
- The Basis and Essentials of Portuguese and reader
- All Purpose Russian for Adults
- Six Days to Shake an Empire
- Italian for Adults
- Mysterious People: An Introduction to the Gypsies, 1965
- The Truth about Columbus and the Discovery of America, 1936
- Handrail and the Wampus; three segments of a polyphonic biogriad, 1931
- James Joyce and the Plain Reader, 1932
- This Human Life (No. 64 in the Thinker's Library)
- "The Haunted Bungalow", London: Associated Newspapers, 1936