The Curious Case of the Copper Corpse (Flavia de Luce #6.5) by AlanBradley

Murder! the letter says, Come at once. Anson House, Greyminster, Staircase No. 3. How can Flavia de Luce resist such an urgent plea? After all, examining a dead body sounds like a perfectly splendid way to spend a Sunday. So Flavia hops upon her trusted bicycle, Gladys, whose rubber tires hiss happily along the rainy road, and arrives at her father’s mist-shrouded old school.