Holt, Richard (2000). "Society and Population 600 - 1300". In Palliser, D. M.; Clark, Peter; Daunton, Martin J. (eds.). The Cambridge Urban History of Britain. Vol. 600–1540. Cambridge and New York: Cambridge University Press. p. 88. ISBN9780521444613.
Bauer, Susan Wise (2013). "Chapter Nineteen: Foreign Relations". The History of the Renaissance World: From the Rediscovery of Aristotle to the Conquest of Constantinople. New York: W. W. Norton & Company. p. 134. ISBN9780393240672.