Byzantium: its ecclesiastical history and relations with the western world. Collected studies, London: Variorum Reprints, 1972, ISBN 0-902089-35-8
Meteora: the rock monasteries of Thessaly, London: Chapman & Hall, 1963. Revised edition: Meteora: the rock monasteries of Thessaly, London: Variorum Reprints, 1975, ISBN 0-902089-73-0
The Byzantine Family of Kantakouzenos (Cantacuzenus), ca 1100–1460: a genealogical and prosopographical study, Washington DC: Dumbarton Oaks, 1968
Church and Society in the Last Centuries of Byzantium (The Birkbeck Lectures in Ecclesiastical History 1977), Cambridge University Press, 1979, ISBN 0-521-07167-4
Byzantium and Venice: A Study in Diplomatic and Cultural Relations, Cambridge University Press, 1988, ISBN 978-0-521-34157-8
Joannes Gennadios, the man. A biographical sketch, American School of Classical Studies at Athens, 1990
A biographical dictionary of the Byzantine empire, London: Seaby, 1991, ISBN 1-85264-048-0
The Immortal Emperor: the life and legend of Constantine Palaiologos, last Emperor of the Romans, Cambridge University Press, 1991, ISBN 0-521-89409-3