R Humphreys was a prolific author of over 50 books, which mainly consist of historical non-fiction. Here is a list of some of his most notable works:
- Age of Revolution (1962) - A history of European revolutions from the late 18th century to the early 19th century, including the French Revolution.
- The Making of Modern France (1971) - A history of France from the middle of the 18th century to the end of the Second World War in 1945.
- The Life of Robert Bruce (1977) - A biography of the 14th century Scottish king Robert Bruce, who led Scotland to independence from England.
- The First Crusade (1985) - A history of the first of the Crusades, which took place from 1095 to 1099 and culminated in the Christian capture of Jerusalem.
- States and Societies of Europe (1989) - A comprehensive survey of European history from the Middle Ages to the present day.
- William Marshal (1982) - A biography of William Marshal (c. 1146–1219), a famous medieval knight who served under five English kings and rose to become one of the most powerful men in England.
- The Normans in England (1974) - A history of the Norman conquest of England and the subsequent Norman dynasty, which ruled England from 1066 to 1154.
- Elizabeth I and Her World (1972) - A biography of Queen Elizabeth I of England and the Elizabethan Age.
- The Vikings (1984) - A history of the Vikings and their raids, conquests and settlements across Europe and beyond.
- London (1991) - A comprehensive history of London from its earliest origins to the modern day.
- The Origins of the English Parliament (1988) - A detailed history of the development of the English Parliament from its early origins to the reign of Edward I (1272-1307).
- **Oxford* (1988) - A history of the city of Oxford from its origins to the modern day, with a focus on the university and its role in English intellectual history.
- The Tudor Age (1990) - A survey of the Tudor period of English history, from the reign of Henry VII to the death of Elizabeth I.