The Wandering Widow — Sarah J. Waldock

When Napoleon Bonaparte is exiled to Elba, widow Letty Grey takes advantage of the peace to visit France. Escorted by an old friend of her husband's, sometime British spy, Viscount Leomer Byesby, Letty finds the trip riven with surprises from start to finish. She stays with Lillias, a former actress, who is dying from consumption, and both Leomer and Letty work hard to make her last days full of happiness. With Napoleon's unexpected return to Paris, things are no longer safe for a British spy, nor his companions. As an old adversary of Leomer's closes in, Letty and Leomer must ultimately take to the catacombs of Paris.