Bowersock, G. W.; Brown, Peter; Grabar, Oleg (1999). Late Antiquity : A Guide to the Postclassical World. Cambridge, Mass.: Belknap Press of Harvard University Press. ISBN0-674-51173-5. OCLC41076344.
Luttwak, Edward (2009). "Bulghars and Bulgarians". The Grand Strategy of the Byzantine Empire. Cambridge, Mass.: Belknap Press of Harvard University Press. ISBN978-0-674-05420-2. OCLC648760614.