Bulgars, Croats, and Serbs

• Curta 134-147
•  John V. A. Fine, "Croats and Slavs: theories about the historical circumstances of the Croats' appearance in the Balkans," Byzantinische Forschungen 26 (2000), 205-218 (Pack)
  
• Theophanes, Chronographia, a. M. 6171
  
• see also a passage from the Chronicle of Monemvasia
• see a map of the most important sites

Early medieval Bulgaria

• Curta 81-90 and 147-166
• Florin Curta, "Qagan, khan, or king? Power in early medieval Bulgaria (seventh to ninth century)," Viator 37 (2006), 1-31 (available online)
• Uwe Fiedler, "Bulgars in the Lower Danube region. A survey of the archaeological evidence and of the state of current research," in The Other Europe in the Middle Ages. Avars, Bulgars, Khazars, and Cumans, ed. by Florin Curta (Leiden/Boston, 2008), pp. 151-236 (Pack).
• see a gold medallion of Omurtag
• visit Pliska, the first capital of medieval Bulgaria, and the site of the Madara Horseman