Agatha Christie 
By the Pricking of My Thumbs [EPUB ebook] 

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…