Fueter, Eduard (1922). World history, 1815–1930. United States of America: Harcourt, Brace and Company. pp. 25–28, 36–44. ISBN1-58477-077-5. Great Powers Congress of Vienna.
Henry Davis: Medieval Europe. Williams and Norgate, London 1911, p. 55: "These crowded years of war leave the Frankish Empire established as the one great power west of the Elbe and Adriatic."
Daniel Ziemann: Das Erste bulgarische Reich. Eine frühmittelalterliche Großmacht zwischen Byzanz und Abendland. (German: An early medieval great power between Byzantium and the Occident) In: Online handbook on the history of South-East Europe. Volume I Rule and politics in Southeastern Europe until 1800. Published by the Institute for East and Southeast European Studies of the Leibniz Association, Regensburg 2016
