Ten Years Later by Alexandre Dumas

In this continuing sequel to The Three Musketeers, d'Artagnan discovers Belle-Isle is being fortified and the engineer ostensibly in charge is Porthos. The blueprints show Aramis' handwriting. Despite his friends, d'Artagnan hides the true reason for his presence. Aramis, suspicious of d'Artagnan, sends Porthos back to Paris to warn Fouquet, whilst tricking d'Artagnan into searching for Porthos around Vannes. Porthos warns Fouquet in time, and he cedes Belle-Ile to the king, humiliating Colbert. On returning from the mission, d'Artagnan is made Captain of the King's Musketeers.