Cinq Avril: The Heir of da Vinci (Cinq Avril #1)

On April 5th, 1514, a mysterious noblewoman leaves a newborn bearing only an unusual golden necklace at the gates of the Château du Clos Lucé in Amboise, France. Named "Cinq Avril" by the cooks who care for him, and taken under the wing of another resident of the castle, a certain Leonardo da Vinci, the child grows in energy and knowledge as he's instructed in a multitude of secrets. When his mentor dies, the young man discovers a posthumous letter explaining that he has the power to change the course of history if he can unravel the enigma of his birth.