By the Pricking of My Thumbs (A Tommy & Tuppence Mystery) - Agatha Christie

An old woman in a nursing home speaks of a child buried behind the fireplace... When Tommy and Tuppence visited an elderly aunt in her gothic nursing home, they thought nothing of her mistrust of the doctors; after all, Ada was a very difficult old lady. But when Mrs Lockett mentioned a poisoned mushroom stew and Mrs Lancaster talked about `something behind the fireplace', Tommy and Tuppence found themselves caught up in an unexpected adventure involving possible black magic...