If only Titian had left us the scathing letters and soul-baring sonnets of his contemporary Michelangelo, whose image as a tortured genius has enchanted biographers since the Renaissance. In their place we have just a smattering of Titian’s business correspondence and a few official statements. His friendship with the ferocious satirist and pornographer Pietro Aretino is the stuff of speculation. So too are his relations with the models and companions who appear in masterpieces like the “Venus of Urbino,” which transforms one of Venice’s highest-priced courtesans into a goddess of love, taunting us with her half-smile while resting her hand in a forbidden realm.