These books tell the story of previous outreaches to Tibetan peoples.  Many are scanned copies of out of print books from the Internet Archive.

With the Tibetans in Tent and Temple, by Susie Carson Rijnhart (1901)

Shelton of Tibet, by Flora Beal Shelton (1923)

Life Among the People of Eastern Tibet, by Dr. A.L. Shelton (1921)

Pioneering in Tibet : A Personal Record of Life and Experience in Mission Fields, by Albert Shelton (1921)
 
Tibet and the Tibetans, by Graham Sandberg (1905)
 
Outposts: or, Tibetan Border Sketches, by David Ekvall (1907)
 
Our West China Mission, The Missionary Society of the Methodist Church of Canada

Faithful Echo, by Robert Ekvall (1960)

Fields on the Hoof: Nexus of Tibetan Nomadic Pastoralism, by Robert Ekvall (1968)