I do like Lord John, and I think he works well in novella format. Each of these tales has at least some nominal paranormal frame of reference. I'm not sure I would call them mysteries, however. The second story is my favorite. It was well developed, and Diana's wit and humor is allowed to shine. The third story is the most introspective, and lets us get to know John and his family a little better. Those who were put off by the sex scenes in Brotherhood of the Blade won't find anything to bother them here.
Diana Gabaldon delivers three tales of war, intrigue, and espionage that feature one of her most popular characters: Lord John Grey.
In Lord John and the Hellfire Club, Lord John glimpses a stranger in the doorway of a gentlemen’s club—and is stirred by a desperate entreaty to meet in private. The rendezvous forestalled by a sudden murder, Lord John will wade into a maze of political treachery and a dangerous, debauched underground society. . . . In Lord John and the Succubus, English soldiers fighting in Prussia are rattled by the nocturnal visitations of a deadly woman who sucks life and soul from a man. Called to investigate the night-hag, Lord John finds a murdered soldier and a treacherous Gypsy, and comes to the stark realization that among the spirits that haunt men, none frighten more than the specters conjured by the heart. . . . In Lord John and the Haunted Soldier, Lord John is thrust into the deadly case of an exploding battlefield cannon. Wounded in the same battle, Lord John is called to testify and soon confronts his own ghosts—and the shattering prospect that a traitor is among the ranks of His Majesty’s armed forces.
See: Lord John and the Private Matter