The short 20 minute video that you'll find below is a good overview of the birth and history of Christianity in Thailand. It was prepared for the 2003 celebration of 175 years of Protestant Christianity in Thailand, counting from 1828 when Karl Gutzlaff and Jacob Tomlin arrived as the first two Protestant missionaries.
There is also a Thai-language book that goes along with the video, called "๑๗๕ ปี พันธกิจคริสต์ศาสนาโปรเตสแตนต์ในประเทศไทย, ค.ศ. ๑๘๒๘-๒๐๐๓" (175 Years of Protestant Christian Ministry in Thailand, 1828-2003). The book is currently out of print but should be able to be found in libraries of seminaries and bible colleges in Thailand.
If you don't see a video above, click here to watch it on Youtube.