U.S. military intelligence reports: Japan, 1918-1941
"The records consist of United States military intelligence reports relating to military, political, and economic affairs in Japan. The intelligence reports are based upon interviews with military and government officials, surveillance of activities of political parties, and extensive eyewitness coverage of military action. The reports reflect both the complexity and the global significance of events in Japan between the world wars."
British documents on foreign affairs-reports and papers from the foreign office confidential print. Series East Asia
Part I 1860-1914 ; Part II 1914-1939 ; Part III 1940 -1945
Papers relating to the foreign relations of the United States. Japan: 1931-1941
Washington: U.S. Govt. Print Off, 1943.
One Year of Japanese Occupation of Manchuria. Nanking.
International Relations Committee. China: International Relations Committee 
Confidential U.S. Diplomatic Post Records Japan 1914- 1941
Frederick, MD : University Publications of America, 1983.