Reviews say that the "theory" is advanced that the Mona Lisa is a self-portrait. Since the banker who bought it described his check as "payment for my wife's portrait" this does suggest a rather illicit situation...
No review that I've seen by anyone knowing much about art, technique or history, gives much credibility to his "conspiracies," although most credit him with having written a "fine fantasy."
Sort of like Patrick O'Brien's 20 novels about "life on the roarin' main" - by a man who was not only not Scots-Irish as he maintained for some 70 years, but who had never been to sea. (See article Master Storyteller in the Decemer 2003 Smithsonian.) Still reportedly good, and very "authentic sounding," tales.