One way that I build on the entries in "An English-Language Bibliography of Materials Related to Christianity in Thailand," is by selecting authors of current entries and searching online for additional citations by that author. Sometimes all I get is nothing, but sometimes the Web contains fascinating new sources. Such is the case with Dr. Carle Clark Zimmerman (1897-1983), a Harvard sociologist who spent time in Siam back in the 1930s working with the American Presbyterian Mission. He seems to have worked primarily with Bertha Blount McFarland, a former member of the mission who married George Bradley McFarland. She remained very active in Christian work in Bangkok after her marriage. Zimmerman conducted sociological studies for the mission, and the most important result of his work was the book, Siam: Rural Economic Survey, 1930-31. That book does not appear in the bibliography because it is about Siam generally and not about Christianity in Siam at all.
Zimmerman, however, did produce some unpublished work relevant to Thai Christianity that he co-authored with Bertha McFarland. It turns out that the University Archives & Special Collections division of the University of Saskatchewan Library contains a manuscript collection of Zimmerman materials (finding aid) that includes a couple of interesting items regarding his work in Siam: the first is a diary that Zimmerman kept of a trip to Chiang Mai in December 1930-January 1931; and the second is a 271 page unpublished manuscript entitled, "Christian Missions in Siam: A Study in Oriental Culture." I haven't included the diary in the bibliography because I'm not sure of its contents, but the unpublished manuscript is obviously relevant and an unexpected find.
Knowing the quality of Zimmerman's work, it is very likely that his study of the missions in Siam is a very helpful document. It could be valuable in a couple of ways. First, the missionary sources and other English-language sources for the study of Thai church history are relatively meager for the 1930s; second, Zimmerman brought an academic expertise to his research, which is rare for that era.