A new biography of Zane Grey, whose prolific output of Western novels made him the best-selling American writer in the first forty years of the last century, quotes a line from one of his books: “All the cowboys had secrets.”

There are two ways to look at Giacomo Casanova, the 18th century Venetian whose legend retains a hold on our imagination two centuries after his death. There is the real Casanova, whose eleven volumes of memoirs seem barely sufficient to contain his exploits as a diplomat, a politician, and a spy; a musician and a writer; a Freemason and a cleric; and a swordsman, in both senses of the word. That his biography isn’t entirely trustworthy only adds to his allure.