The Kingdom: FARGO Adventures #3, Clive Cussler

Whether it's lost treasure or missing persons, the Fargos find themselves in a heap of trouble every time. . . 

When Texas oil baron Charlie King contacts Sam and Remi Fargo he has an unusual request.  He hired an investigator - and a good friend to the pair - to locate his missing father in the Far East.  But now the investigator has vanished.  Would Sam and Remi be willing to look for them both?

Though something about the request doesn't quite add up, Sam and Remi agree to help out. 

It's a journey that takes the Fargos to Tibet, Nepal, Bulgaria, India and China.  They get mixed up with the black market fossils, a centuries old puzzle chest, the ancient Nepali kingdom of Mustang, a balloon aircraft from a century before it's time. . . and an extraordinary skeleton that might turn the history of human evolution on its head.  And not  forgetting a few unfriendly people with guns and itchy trigger fingers. . .