1. Beševliev , V. 1963 Die protobulgarischen Inschriften
2. Beševliev , V. 1981 Die protobulgarischen Periode der bulgarischen Geschichte
3. Browning , R. 1975 Byzantium and Bulgaria: a comparative study across the early medieval frontier
4. Qagan, Khan or King? Power in early medieval Bulgaria (seventh to ninth century);Curta;Viator,2006
5. Bulgars In The Lower Danube Region. A Survey Of The Archaeological Evidence And Of The State Of Current Research