A recommendation on Justinian's reign would be "The Cambridge Companion to the Age of Justinian" by Michael Maas. However, if you want an in-depth analysis on the Byzantium empire, a good book would be "Byzantium in the Iconoclast Era, c.680-850: A History" by Leslie Brubaker, John Haldon.

In my opinion, Belasarius is not as well known as other generals such as Napoleon and Alexander the great as most of his battles are mostly defensive or just recovering lost territory, thus not making a distinct impression on the people. On the other hand, Alexander the great manage to humble the Persian empire, which was thought to be a superpower at that time, he conquered so many lands that his army mutinied when he wanted to continue and he died before any action that could harm his reputation was made, thus he became a hot topic with many ancient historians such as Diodorus of Sicily, Quintus Curtius Rufus and Arrian. Thus he became well known. As for Napoleon, he was a radical and his conquests and bold and unique tactics and great victories had gotten the attention of the whole of Europe which could be consider as 75% of the whole world then, thus people would remember him as many people during that period would have heard of him and legends, stories and literacy works on him would be created, thus making him well known.