Procopius was an official chronicler during a particularly important period in Byzantine history. Emperor Justinian the Great and his forceful wife Theodora were on the thrones, and the great general Belisarius reconquered Libya and Italy (these conquests would become the Exarchates, the former of which furnished the empire with the slayer and successor of Phocas, namely Heraclius).
However, whilst writing rather obsequious official histories Procopius was also working on The Secret History, to be published after his death. It’s an unremitting litany of poison, contempt and loathing, heaped mostly upon Justinian, Theodora and (to a lesser extent) Belisarius.
Belisarius is portrayed as a weak-willed naïve man entirely under the thumb of what could politely be described as a frisky wife with a substantial carnal appetite. There are certain elements of praise for Belisarius’ military ability but he is mostly criticised for a weak and pitiful character.
For Justinian, described as a demon (once literally), and his wife there is no such limited counterpoint to their flaws. The emperor is a creature of absolute vice, abandoning his friends willy-nilly, succumbing to a wife unfit for her position and taxing the Byzantines into poverty whilst hurling their gold at barbarians who did not even ask for it.
Justinian is not painted as an idiot incapable of rule, but as an intelligent man who wilfully sought to weaken the empire, damage its people and accrue wealth (which was then uniformly squandered) by laying false charges against the prosperous and confiscating their property.
The very constancy of the attacks upon Justinian’s character (leaving aside the references to him as a demonic creature) damage Procopius’ own case, as they make him appear entirely one-sided. However, the very fact that such passionate hatred (remembering that the author risked his life writing the book) was engendered in him suggests either a vendetta or a grievous abuse of mankind generally of which Procopius happened to be one victim (or witness).
The Secret History is short, just over 120 pages, and covers a time (the 6th century AD) not especially well-known by people not into Byzantine history. I wouldn’t recommend it to people who hadn’t already read a more general history of the time (and thus had a feel for the kind of state the world and Byzantium was in). I thought it reasonably enjoyable, but no more than that.