Nature of the Crime by Cara Devlin

The Bow Street Duchess Mystery series continues with the sixth Audrey Sinclair and Hugh Marsden investigation…

When a fellow passenger is found dead on Audrey’s packet ship from France, a mysterious note on the body points to her as the killer. Detained in Dover for an inquest that is being led by a prejudiced magistrate, she is joined by trusted friends and family—and the newly minted viscount, Hugh Marsden—to prove her innocence.
But when it’s discovered the murdered man was a private inquiry agent, hired to follow the duchess while she traveled the Continent, they enter a race to find out who hired him and why—and what secrets the true killer intends to expose before sending Audrey to an early grave.