The Vicomte de Bragelonne by Alexandre Dumas

In this continuing sequel to The Three Musketeers, d'Artagnan resigns as captain of the Musketeers as he perceives the young king Louis XIV as weak-willed. He resolves to aid the exiled Charles II to retake the throne of England, unaware that Athos is attempting the same. With their assistance Charles II is restored to the throne and d'Artagnan is rewarded richly.