The Candy Factory Mystery — Gertrude Chandler Warner

When Grandfather's friend Mrs. Winkles needs some extra workers at her candy factory, the Boxcar Children are thrilled to help. What could be more fun than spending their days making candy? But working in the factory isn't as sweet as they expected. Workers are quitting, machinery is breaking down, and mysterious threatening messages are appearing on the candy hearts. It looks like someone is sabotaging Mrs. Winkles's candy!