The Listening Eye — Patricia Wentworth

Miss Silver paid close attention to the strange tale of sensible, middle-aged Miss Paulina Paine. Though totally deaf, Miss Paine could lip-read so well that strangers never suspected her handicap--or guessed she could eavesdrop with uncanny accuracy. She had been, she told Miss Silver, on the far side of a spacious art gallery, but her "listening eye" was not mistaken: she distinctly "heard" two men planning both a theft and a murder.

Miss Paine hadn't a clue as to the victim. But before the week was up, her account of the two conspirators would lead Miss Silver to Merefields, a famous English country estate where a diamond necklace was missing, a young man had died, and -- unless Miss Silver could stop him -- a killer was about to strike again..