Enola Holmes and the black barouche by Nancy Springer

When Miss Letitia Glover shows up on Sherlock’s doorstep, desperate to learn more about the fate of her twin sister, it is Enola who steps up. Letitia Glover is convinced her sister is still alive and that her sister’s husband is lying. Enola discovers that this is not the first time a wife of the Earl has met an untimely death. Could it be that the secret to the fate of the missing Felicity is tied to a mysterious black barouche that arrived at the Earl’s home in the middle of the night? To uncover the secrets held tightly within the Earl’s hall, Enola is going to require help – from Sherlock, from the twin sister of the missing woman, and from an old friend, the young Viscount Tewkesbury, Marquess of Basilwether!