Happy Hollisters and the Swiss Echo Mystery

Business had originally brought the Hollister family to Europe. Then a new and exciting mystery led Pete, Pam, Holly, Ricky, Sue, and Mrs. Hollister on a thrilling chase while Mr. Hollister returned to America.
High in the Swiss Alps, a suspicious stranger in a cable car dropped a large black key which the Hollisters found. An insurance investigator named Johann Meyer was following the stranger, an international jewel thief. Mr. Meyer told the Hollisters about his dangerous case-to find a large diamond that had been stolen from Holland. When Mr. Meyer's small plane was reported missing over the mountains, the Hollisters decided to help.
The trail took them to a small village high in the mountains where Holly thought she saw the stranger again. But the man escaped before they could catch him. Then Pete and Pam discovered a ruined chalet which yielded another clue-a round metal disc that they later identified as a diamond saw. But who, they wondered, was blowing the long Alpine horn which echoed off the mountain peaks and where did the medallion necklace that Pam had found fit into the case?
Clue piles on clue as the irrepressible Hollisters track down each new lead, learn about Swiss life and customs, rescue Mr. Meyer, and recover a valuable diamond.
Paperback; 174 pages with 20 illustrations