I give it to you : a novel / Valerie Martin.

Enhanced descriptions from Syndetics:

A timeless story of family, war, art, and betrayal set around an ancient, ancestral home in the Tuscan countryside from bestselling novelist Valerie Martin.

When Jan Vidor, an American writer and academic, rents an apartment in a Tuscan villa for the summer, she plans to spend her break working on a novel about Mussolini. Instead, she finds herself captivated by her aristocratic landlady, the elegant, acerbic Beatrice Salviati Bartolo Doyle, whose family has owned Villa Chiara for generations. Jan is intrigued by Beatrice's stories of World War II, particularly by the tragic fate of her uncle Sandro, who was mysteriously murdered in the driveway of the villa at the conclusion of the war. Day by day, Beatrice makes Jan privy to her family history.

As years go by and the friendship is sustained by infrequent meetings, Jan finds she can't resist writing Beatrice's story. But as she works on the novel, it becomes clear that the villa itself is at risk and that Beatrice is incapable of saving it. Jan understands that she is telling the story of a catastrophe her friend might prefer to conceal. She presses on.