Forbes, Nevill; Arnold J. Toynbee, D. Mitrany, D.G. Hogarth. "The Rise and Fall of the Second Bulgarian Empire: 1186–1258". The Balkans: A History of Bulgaria, Serbia, Greece, Romania and Turkey. Globusz Publishing. ISBN 0-404-02457-2. http://www.globusz.com/ebooks/Balkans/00000018.htm.
