"Prokopios is the essential source for the reign of Justinian. It is available in seven volumes in the Loeb Classical Library. The translation is old, but complete and remains good today. All of these volumes have the Greek text."
"A recent translation of Prokopios' 'Anekdota' by the eminent Anthony Kaldellis. Includes some other useful texts from Justinian's legal works as well as contextual snippets from Prokopios' 'History of the Wars'."