The Sweetness at the Bottom of the Pie (Flavia de Luce #1) by Alan Bradley

It is the summer of 1950–and at the once-grand mansion of Buckshaw, young Flavia de Luce, an aspiring chemist with a passion for poison, is intrigued by a series of inexplicable events: A dead bird is found on the doorstep, a postage stamp bizarrely pinned to its beak. Then, hours later, Flavia finds a man lying in the cucumber patch and watches him as he takes his dying breath.