Ziemann, Daniel. Das Erste bulgarische Reich. Eine frühmittelalterliche Großmacht zwischen Byzanz und Abendland (English: The First Bulgarian Empire: 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.
Ziemann, Daniel. Das Erste bulgarische Reich. Eine frühmittelalterliche Großmacht zwischen Byzanz und Abendland (English: The First Bulgarian Empire: 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.