The Married Man by Edmund White

In Edmund White's most moving novel yet, an American living in Paris finds his life transformed by an unexpected love affair.
Austin Smith is pushing fifty, loveless and drifting, until one day he meets Julien, a much younger, married Frenchman. In the beginning, the lovers' only impediments are the comic clashes of culture, age, and temperament. Before long, however, the past begins to catch up with them. In a desperate quest to save health and happiness, they move from Venice to Key West, from Montreal in the snow to Providence in the rain. But it is amid the bleak, baking sands of the Sahara that their love is pushed to its ultimate crisis.