The Secret History was written by Procopius in the 6th century A.D. It is a juicy and scandalous exposé of the Roman Emperor Justinian I and his wife, the Empress Theodora.
Justinian had charged Procopius with writing the official history of the his reign. He did this, but also kept a secret history, which was only published after his death. He revealed his harsh opinons of the Emperor's character, and his disapproval of the Empress, who had been an actress before her marriage.
About Justinian, Procopius has this to say:
This Emperor, then, was deceitful, devious, false, hypocritical, two-faced, cruel, skilled in dissembling his thought, never moved to tears by either joy or pain, though he could summon them artfully at will when the occasion demanded, a liar always, not only offhand, but in writing, and when he swore sacred oaths to his subjects in their very hearing. Then he would immediately break his agreements and pledges, like the vilest of slaves, whom indeed only the fear of torture drives to confess their perjury. A faithless friend, he was a treacherous enemy, insane for murder and plunder, quarrelsome and revolutionary, easily led to anything, but never willing to listen to good counsel, quick to plan mischief and carry it out, but finding even the hearing of anything good distasteful to his ears.
But The Secret History is mostly notable for its descriptions of Theodora's amusing iniquity. Her "acting" skills included reclining naked except for a ribbon on stage, covered with grains of barley which geese pecked off her body. Procopius describes her gleeful participation in gang bangs, and her enjoyment of seducing young boys.
In short, a good read, maybe even better than The Twelve Caesars if you like scandalous ancient history.