Hallowe'en Party (Hercule Poirot #39) by Agatha Christie

A teenage murder witness is drowned in a tub of apples... At a Hallowe'en party, Joyce—a hostile thirteen-year-old—boasts that she once witnessed a murder. When no-one believes her, she storms off home. But within hours her body is found, still in the house, drowned in an apple-bobbing tub. That night, Hercule Poirot is called in to find the 'evil presence'. But first he must establish whether he is looking for a murderer or a double-murderer...