The Nine-Diamond Ring is the second novel in the Louisa Moore Mysteries series, following The Accidental Detective. Louisa Moore and Sam Goff have been dating regularly for about a year; Louisa is hoping that Sam will soon ask her to marry him. Two weeks before Christmas, Louisa's cousin Estelle Hartley sees Sam in a jewelry store and overhears him talking to a salesman about the nine-diamond ring he had just purchased for a very special friend. After Estelle tells Louisa that Sam will surprise her with a nine-diamond engagement ring on Christmas Day, Louisa begins thinking about and preparing for her marriage to Sam.