Isabella d'Este, the marchioness of Mantua, was a collector of antiquities, a patron of art, and one of the most vivid personalities of the Italian Renaissance. Her artistic relationship with Leonardo da Vinci (1452-1519) is charted through the letters that they exchanged over the course of about six years. Beginning in late 1499, Leonardo spent several months in Mantua, where he met Isabella and produced a finished portrait drawing of her. In the years that followed, the marchioness wrote to the artist to ask him to undertake other paintings and projects. Though little came of these...