- EARLY MIDDLE AGES, 500-1000 (Sources in Western Civilization)
- Two Churches: England and Italy in the Thirteenth Century, With an additional essay by the Author.
- Rome before Avignon: A Social History of Thirteenth-Century Rome
- The Prophetia Merlini of Geoffrey of Monmouth: A Fifteenth-Century English Commentary (Speculum Anniversary Monographs Eight)
- A New World in a Small Place: Church and Religion in the Diocese of Rieti, 1188-1378