Mystery in the Sand - Gertrude Chandler Warner

The Boxcar Children are spending the summer in a mobile home on the beach. One morning Benny finds a valuable gold locket in the sand and decides to try to find its owner. Soon, the kids are caught up in a puzzle that involves shadowy figures on the beach and the mysterious Miss Smith, who seems to be alone in the big Tower House nearby. Living in a seaside mobile home, the Aldens unravel the secret of two secluded women.