Salai is the name that Leonardo da Vinci liked to use, as is documented in his writings from 1494 onward, for the boy whom he had taken into his home four years earlier, when he was just ten years old, and who lived with him for almost a quarter of a century: first as a shop boy and then as his assistant and model, as well as becoming, according to what the sources tell us, his pupil and companion in life. In reality we know very little about him, except for this work which is being presented to the general public for the first time today. The only one in which his name is indicated with precision. A document that could therefore throw some light on a figure still shrouded in mystery and add an important element to the history of art. The author of the volume, Maurizio Zecchini, offers a moving and gripping historical account, as well as presenting, with the assistance of a team of experts, the results of the numerous analyses carried out during the restoration of the work.