If your library is full of old National Geographic Magazines, then maybe try to hunt down these old articles published about Yunnan.
The Land of the Crossbow
One of George Forrest's few publications.
Joseph Rock (1922-1935)
Our Man in China
Joseph F. Rock pushed of his pedestal.
Banishing the Devil of Disease Among the Nashi
Weird Ceremonies Performed by an Aboriginal Tribe in the Heart of Yunnan Province
Through the Great River Trenches of Asia
National Geographic Society Explorer Follows the Yangtze, Mekong, and Salwin Through Mighty Gorges