Nikolov, Georgi N. “State Organisation and Power Hierarchy in the Bulgarian Empire (927–969)”. The Bulgarian State in 927-969. The Epoch of Tsar Peter I: No. XXXIV, edited by Mirosław J. Leszka, Lodz University Press, 2018, pp. 257-68, https://doi.org/10.18778/8142-115-7.14.