* "A History of the Crusades" (1955-1989), a multi-volume collaborative project of which he was the principal editor. This monumental work is considered the definitive reference on the Crusades.
* "The Papacy and the Levant (1204-1571)" (1978), which focuses on the relationship between the papacy and the Eastern Mediterranean during the Middle Ages.
* "The Byzantine Empire" (1977), a concise and informative overview of Byzantine history for a general audience.
* "Christian Attitudes Towards the Jews in the Middle Ages" (1940), a groundbreaking study exploring the complex and often fraught relationship between Christians and Jews in medieval Europe.
He also wrote numerous articles and essays on various aspects of medieval history. His contributions have significantly shaped our understanding of the Byzantine Empire and the Crusades.