Around a few entries in Leonardo's notebooks Konigsburg constructs the character of his young apprentice Salai and fabricates an answer to two questions. She begins, "Why, people ask, why did Leonardo da Vinci choose to paint the portrait of the second wife of an unimportant Florentine merchant? . . . Why, they ask, why?" But it's the second question, which surely fewer "people ask," that gets a fuller answer here: "Why, people ask, why did Leonardo da Vinci put up with this liar, this thief, this Salai? Why for so long?. . . Why?" In the words of Beatrice, the plain but inwardly beautiful young duchess who shares the boy's sense of fun and mischief, Salai represents the one wild element--irreverent, unserious--that the too self-conscious Leonardo needs for greatness. And when, at the very end, Mr. Giaconda brings his wife to the absent artist's studio, Salai recognizes her as that same "unimportant, importantly unserious wild thing" whose portrait would not only recall the now dead Beatrice but would also be the perfect insult to Beatrice's shallow, arrogant sister Isabella who was begging to be painted herself. The Mona Lisa then is, at least in part, Salai's revenge on a silly snob; the master himself is remote throughout the story though through his assistant's eyes we are shown a number of his projects. We also see him on occasion throw back his head and laugh at Salai's dismissal of men of learning and station--though the boy's "those guys. . ." and (of the duke) "who does he think he is" seem fresh and spunky only in contrast with the complacently polished speech employed by everyone else including the author. At this civilized midcult level Konigsburg is successful, mixing palatable art history with mildly ingenious conjecture, and she probably should not be faulted for not having a Salai of her own.