The December book is Silent Night by Mary Higgins Clark.

"When Catherine Dornan's husband, Tom, is diagnosed with leukemia, she and their two young sons travel with him to New York during the holiday season for a lifesaving operation. On Christmas Eve, hoping to lift the boys' spirits, Catherine takes them to see Rockefeller Center's famous Christmas tree; while there, seven-year-old Brian notices a woman taking his mother's wallet. A St. Christopher medal tucked inside the wallet saved his grandfather's life in World War II, and Brian believes with all his heart that it will protect his father now. Impulsively, Brian follows the thief into the subway, and the most dangerous adventure of his young life begins..."