Man in the iron mask
There are several books and movies about this man. The best known of these are probably the story by Alexandre Dumas called "The Vicomte de Bragelonne" (which is the third volume of his work  The Three Musketeers), the story The Twins, by Victor Hugo and The Iron Mask by Marcel Pagnol. Dumas also gives a summary of the theories about the prisoner, which were common at his time.