The Wars of Justinian (Greek: Ὑπὲρ τῶν πολέμων λόγοι, Hypèr tōn polémon lógoi; Latin: De Bellis) by Procopius of Caesaria is a major source for the history of the Byzantine Empire in the mid-sixth century CE. The first two book deal with campaigns against the Sassanids (The Persian War) . Books three and four cover Belisarius' campaign against the Vandals in North Africa (The Vandal War). The remaining books record the struggle to regain Italy from the Ostrogoths (The Gothic War).
Free online texts
Loebulus. L048 - Procopius I: History of the Wars, Books 1-2. (Persian War) | L081 - Procopius II: History of the Wars, Books 3-4. (Vandalic War) | L107 - Procopius III: History of the Wars, Books 5-6.15. PDF of public domain Loeb edition in Greek and English.
Wikisource: History of the Wars, translated by H.B. Dewing. Multiple formats.
Duke.edu: The Dates of Procopius' Works: A Recapitulation of the Evidence, by J.A.S. Evans (1997).
Encyclopedia Iranica: Procopius.
RobertGraves.Org: Count Belisarius and Procopius’s Wars, by Peter G. Christensen, Gravesiana: The Journal of the Robert Graves Society, Vol.IV, No 1 (2014).
The Great Conversation: Further reading at Tom's Learning Notes
Procopius: The Secret History.
Ancient Greek resources: Learn to read Greek classics in the original.