The Secret Panel (Electronic book text)

Another exciting mystery begins for Frank and Joe Hardy when they help a stranger who has had an accident with his car. The man introduces himself as John Mead, owner of a nearby estate. After he continues on his way, Frank finds an odd-looking house key which belongs to Mead. But when the Hardys try to return it, they learn that John Mead died five years ago! They are even more amazed when they find that the intricately carved doors in the dead man's deserted mansion have no visible knobs or keylocks.