The Man in the Iron Mask. The d'Artagnan Romances, Book 4

Thirty-five years after the musketeers first drew together, Aramis is a bishop with a plan, Porthos does not understand it, Athos is grieving, and d'Artagnan serves a king who has a secret in the Bastille. Dumas ran a fiction factory and outsold all of France; this is the end of the cycle, and he wrote the fall of his own heroes without flinching.