The Baltic Citadel - Patrick O'Brian - book

If anyone had told Jack Aubrey - a Royal Navy captain - and his sidekick Stephen Maturin - a doctor, naturalist, and secret agent - that one day they would face perils on land greater than those of the ocean, they would never have believed it... Yet that's what's happening to them today. They're forced to leave Halifax as quickly as possible, where they planned to celebrate the Navy's victory over an American schooner. Jack, to escape a schemer who has set her sights on him. Stephen, to elude the French and American intelligence agents pursuing him. In England, Stephen is entrusted with a most delicate mission: to land on a fortified island in the Baltic held by a Catalan brigade in the service of the French and convince their leaders to submit. Naturally, he drags Jack into this adventure.