Reading the Middle Ages: Sources from Europe, Byzantium, and the Islamic World, Barbara H. Rosenwein, University of Toronto Press, 2013, ISBN 1442606029, p. 160.
Southeastern Europe in the Middle Ages, 500 – 1250, Florin Curta, Cambridge University Press, 2006, ISBN 0-521-81539-8, pp. 162 – 163.
A Concise History of Bulgaria, R. J. Crampton, Cambridge University Press, 2005, ISBN 1-139-44823-4, p. 270.
