This is book 12 of the "Aubrey-Maturin" series.

This note contains spoilers for the book.

They're off in the Surprise as a privateer, with an admiralty mission coming up. Actually, most of the action takes place on a trial cruise, during which they manage to take a hugely valuable prize, 4 other prizes, and a French frigate.

This is the book where the smugglers and privateers of Shelmerston, including the Sethians, are introduced.