Death Comes to London(Kurland St. Mary Mysteries‪)‬

Murder makes a debut on the London social scene in this Regency-era mystery by the author of Death Comes to the Village.
With the reluctant blessings of their father, the rector of Kurland St. Mary, Lucy Harrington and her sister Anna leave home for a social season in London. At the same time, Lucy's special friend Major Robert Kurland is summoned to the city to accept a baronetcy for his wartime heroism.
Amidst the dizzying whirl of balls and formal dinners, the focus shifts from mixing and matchmaking to murder when the dowager Countess of Broughton, the mother of an old army friend of Robert, drops dead. When it's revealed she's been poisoned, Robert's former betrothed, Miss Chingford, is accused, and she in turn points a finger at Anna. To protect her sister, Lucy enlists Robert's aid in drawing out the true culprit.
But with suspects ranging from resentful rivals and embittered family members to the toast of the ton, it will take all their sleuthing skills to unmask the poisoner before more trouble is stirred up . . .