The reign of Heraclius coincides with the early Islamic conquests, so you'll probably be interested in these works:

• Byzantium and the Early Islamic Conquests, by Walter Kaegi (1992)

• Muslim Expansion and Byzantine Collapse in North Africa, by Walter Kaegi (2010)

Kaegi also wrote Heraclius Emperor of Byzantium (2003), though I have not read it (and looking on Amazon, it's incredibly expensive).

History of the Byzantine State, by Ostrogorsky (2007) is good. There isn't enough about Heraclius specifically (IMO), but there is a broad context of the time period and sufficient information about Heraclius that it would be helpful.