The purple is the best winding sheet,"" Theodora exclaims to Justinian, and refuses to flee the triumphant rebels storming the gates of the Imperial Palace. Historian Charles Diehl takes her at her word, and purple are the paragraphs he winds her in. Theodora is lucky. Many of the empresses who succeeded her were so pious, pallid or politically misconceived that Mr. Diehl is hard put to drum interest in them, as he admits. Among the thirteen royal ladies he discusses, only three are stand-outs: Theodora; Zoe (a virginal recluse until ascending the throne at 50, at which time she started ripping up husbands like theater tickets); and Anna Comnena, remarkable for astuteness. Theodora, however, runs off with the Liz Taylor award. Only a reader with the soul of a lizard, rocksalt arteries and three coronaries notched on his heart could resist the actress-dancer daughter of the bear-keeper at the Hippodrome who rose through a personal history as sordid as anything by Pierre Louys. But then, she was ""born in Cyprus, the hot, passionate land of Aphrodite,"" and took care of herself like a million-dollar contract. The glister of gold enamelling is never quite lost from any of Mr. Diehl's portraits, e.g., Irene the Peaceful, who had her son's eyes put out by the executioner, then spent Easter in a golden chariot drawn by four white horses, throwing fistfuls of coins to the people. Luxury, elegance, fierce passions-- and out the back window, falling sacks as bodies drop into the Bosphorus. Good reading, with three cigars and a bottle of port.