Word and Power in Mediaeval Bulgaria, Ivan Biliarsky, BRILL, 2011, ISBN9004191453, p. 211.
Reading the Middle Ages: Sources from Europe, Byzantium, and the Islamic World, Barbara H. Rosenwein, University of Toronto Press, 2013, ISBN1442606029, p. 160.
Southeastern Europe in the Middle Ages, 500-1250, Florin Curta, Cambridge University Press, 2006, ISBN0521815398, pp. 162–163.
A Concise History of Bulgaria, R. J. Crampton, Cambridge University Press, 2005, ISBN1139448234, p. 270.
