- A Short History of the Papacy in the Middle Ages (University Paperbacks)
- The Individual and Society in the Middle Ages
- A History of Political Thought: The Middle Ages
- The Medieval Idea of Law, as Represented by Lucas De Penna: a Study in Fourteenth-century Legal Scholarship
- Medieval Papalism: The Political Theories of the Medieval Canonists (Maitland Memorial Lectures, 1948.)