The Man in the Iron Mask by Alexandre Dumas

The Vicomte of Bragelonne: Ten Years Later (Le Vicomte de Bragelonne ou Dix ans plus tard) is a novel by Alexandre Dumas, père. It is the third and last of the d'Artagnan Romances following The Three Musketeers and Twenty Years After. It appeared first in serial form between 1847 and 1850.
The Man in the Iron Mask is the fourth and final volume.