Some Penguin Classics defy the very dignity of the term, and surely the Anecdota, the so-called "Secret History" of the 6th century Byzantine historian Procopius is the best possible example of that curious phenomenon.
It's a thoroughly scurrilous work, a scandal-sheet of gossip and fabrication, written in haste and in total privacy (public attribution would have resulted in the author being flogged to bloody cat-string in the public square), very likely at the same time that it's thirty-something author was also writing the work for which he was famous in his own day: his multi-volume (and quite excellent) history of the military campaigns of the great general Belisarius. Procopius was a secretary on the General's staff during the epic campaigns that briefly re-conquered large swaths of Persia, Africa, and Italy, and his sober, generally reliable account of those early campaigns stands as one of the last glimmerings of the great historical tradition of the Empire before it sank into darkness.
The Secret History is the under-side of all that imperial glory. In this brief account (this is a Penguin Classic you can read in one sitting - and trust me, you'll want to), Procopius centers on the four people at the head of his world: the Emperor Justinian, who's portrayed as a cold, grasping, semi-human monster, his wife the Empress Theodora, who in P's famous accounting becomes a jade and a wanton of such magnitude that she makes Messalina look like Mary Tyler Moore (although she has more of the latter's sense of humor - "When the fancy took her," P recounts, quivering with outrage, "she amused herself by turning the most serious matters into a subject for laughter, as if she were watching a comedy on the stage"). Rounding out the quartet are Belisarius (some of P's loyalty is showing even here: though vilified in the book, Belisarius isn't painted nearly so harshly as the others) and his wife Antonina, here a scheming, murderous, headstrong witch.
Grasping Palace officials, corrupt administrators, brutish army officers, and even a marauding whale (it was an enormous creature, nicknamed Porphyrion, and it attacked shipping all around the Golden Horn for decades - almost certainly a territory-crazed male sperm whale) - all show up and get tarred and feathered, but our author saves his most bitter words for the Imperial couple. He recounts an endless stream of slanders against Theodora, including some that will strike haters of Nancy Reagan as familiar:
For from her earliest years she had herself consorted with magicians and sorcerors, as her whole way of life led her in that direction, and to the very end she put her trust in these arts and made them at all times the ground of her confidence.
Of course the end result of all this vituperation is inevitable: the reader comes away entirely rooting for Theodora and intensely saddened by the allusions Procopius makes several times that she's dead in her grave by the time he's writing his character assassination. Procopius retails every single bit of marketplace gossip about the ruling pair, and throughout this book, he then tries to cloak that gossip in a semblance of historical inquiry:
In view of all this, I, like most of my contemporaries, never once felt that these two were human beings; they were a pair of blood-thirsty demons and what the poets call 'plaguers of mortal men'. For they plotted together to find the easiest and swiftest means of destroying all races of men and all their works, assumed human shape, became man-demons, and in this way convulsed the whole world. Proof of this could be found in many things, but especially in the power manifested in their doings. For the actions of demons are unmistakably different from those of human beings.
Most of his contemporaries thought no such thing, of course, and we can legitimately wonder if Procopius himself did either. The Secret History not only outrages: it revels in outrage. You don't have to read many pages of it before you wonder if the author isn't just plain enjoying himself, relaxing after a hard day's work by sitting at his study table and writing the unwritable. I'd wager more than a few writers do the exact same thing today.
This 1966 Penguin Classic is done by Norwich School legend G. A. Williamson, who renders the whole sordid business in the clearest and (mostly) cleanest of prose, guarding himself always with the acerbity of an Oxford don. "I have resolutely refused," he tells us, "to translate barbaroi by 'barbarians', and I trust that I have used no other words or expressions savouring of translationese."
Williamson supplies the translation; Penguin supplies the edition; I supply the recommendation - it's up to you to supply the healthy skepticism. Armed with it, you'll find this an ancient classic you can't put down.