The Man in the Iron Mask (THE GREAT CLASSICS LIBRARY)

This novel is in fact the third part of a larger novel - The Vicomte of Bragelonne: Ten Years Later - which itself is the last of the three d'Artagnan Romances, the series that begins with The Three Musketeers. It tells a much fictionalized version of the mysterious case of the Man in the Iron Mask, a prisoner who in fact wore a velvet mask and then probably only when seen outside his cell. But it also finishes the tales of the various musketeers, Aramis, Athos, and Porthos, and d'Artagnan.
A number of films have been made from this book, starting with one in Italian in 1909. One of the most recent, in 1998, included Leonardo DiCaprio, Jeremy Irons, John Malkovich, Gérard Depardieu and Gabriel Byrne.