The Golden Tresses of the Dead (Sturmtöchter Saga‪)‬

Flavia de Luce, the twelve-year-old chemist and amateur detective is eager to turn professional. She and her father's valet, Dogger, have founded a detective agency, Arthur Dogger & Associates, and unexpectedly cut into their first case during the revelry at her sister Ophelia's wedding reception.
After an eventful ceremony with a missing best man and spontaneous ventriloquist act, spirits are high as Feely and her new husband head for the towering and beautifully iced wedding cake. But as Feely slices into the first piece, a scream rings out - the bridal cake contains a severed human finger.
Delighted, Flavia wraps the finger in a napkin and whisks it away to her chemical laboratory. By studying the embalmed skin, the indentation of a ring, and the slope of the fingernail, she'll not only be able to determine the identity of the victim - but also point a finger at a killer.