* The Last Viking (1952)
* The Dark Age of the Vikings (1954)
* The Vikings in America (1957)
* The Vikings and Christianity (1961)
* The Vikings in the West (1963)
* The Vikings and the East (1966)
* Vikings at War (1968)
* The Vikings and the World (1971)
* The Viking Achievement (1973)
* The Vikings and the Middle Ages (1975)
Articles
* "The Vikings and the Christianization of Scandinavia" (1951)
* "The Vikings and the Crusades" (1953)
* "The Vikings and the Byzantine Empire" (1955)
* "The Vikings and the Arabs" (1957)
* "The Vikings and the Jews" (1959)
* "The Vikings and the Mongols" (1961)
* "The Vikings and the American Indians" (1963)
* "The Vikings and the Australian Aborigines" (1965)
* "The Vikings and the New World" (1967)
* "The Vikings and the Future" (1971)
Other Works
* Editor, The Viking Sagas (1964)
* Translator, The Saga of the Volsungs (1965)
* Translator, The Saga of the Nibelungs (1966)
* Translator, The Poetic Edda (1968)
* Translator, The Prose Edda (1970)