The Mystery of the Laughing Shadow | 
so-so(1969) Twelfth in the "Three Investigators" series of juvenile mysteries. This one has the boys stumbling across a sinister plot to steal the lost treasure of a fictional Native American tribe. The mystery is mostly transparent, and several things haven't aged well -- most noticably the depiction of Native Americans. It's still the Three Investigators but not one of the better ones.
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