The Crisscross Shadow — Franklin W. Dixon

When a man selling leather goods door-to-door steals the key to their detective father's file cabinet, Frank and Joe Hardy set out to track him down. An odd mark on a key case which the man sold to their mother leads the teenage sleuths to an Indian village, whose chiefs begs them to help him find valuable tribal possessions and the deed for his land. How Frank and Joe find the missing deed and Ramapan treasures, how they prevent the phony leather-goods salesman from carrying out a ruthless scheme and how they help their father solve the top-secret case he is working on for the U.S. government makes exciting reading for all fans of the Hardy boys.