Yün-Nan: : The Link between India and the Yangtze
By far the most comprehensive book on Yunnan from the beginning of the 20th century. Davies was commissioned to investigate the possibilities of building a railway link from British Burma to Yunnan, which was seen as the gateway to China's presumed riches in its hinterland. The book also contains an excellent map, much more accurate than anything before.
Cambridge University Press, Cambridge 1909 BibTeX citation