The Dead in Their Vaulted Arches (Flavia de Luce #6) by Alan Bradley

On a spring morning in 1951, eleven-year-old chemist and aspiring detective Flavia de Luce gathers with her family at the railway station, awaiting the return of her long-lost mother, Harriet. Yet upon the train's arrival in the English village of Bishop's Lacey, Flavia is approached by a tall stranger who whispers a cryptic message into her ear.