The Second Mrs. Gioconda by E.L. Konigsburg

The Mona Lisa... Why did Leonardo da Vinci lavish three years on a painting of the second wife of an unimportant merchant when all the nobles of Europe were begging for a portrait by his hand?
No one knows for sure. But this story of Leonardo, his wayward apprentice Salai, and the Duke of Milan's plain young wife, Beatrice d'Este, may hold the clue to the most famous -- and puzzling -- painting of all time.