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
