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.