The Japan Statistics Bureau (on-line) An amazing source of statistics about Japan and it's all available on-line. For statistics on the number of migrants and their visas status see Chapter 2 of the Statistical Handbook. Want to know the number of North Americans or Oceanians in Japan and on what visa they entered Japan? This is the place to look.
Statistics Japan - Prefecture Comparisons: Another good source. Fascinating graphics. See the maps for where Americans or Brazilians live in Japan.
An Introduction to Japanese Society Fourth Edition (2014) by Yoshio Sugimoto. Best overall introduction to Japan that I found. Well-written, great list of other sources and I recommend it highly.
Global fatigue: Transnational markets, linguistic capital, and Korean-American male English teachers in South Korea, Journal of Sociolinguistics (2012) by J. Cho. Migrants from multi-cultural and multi-racial countries can have ancestors from all over the world. I shouldn't have to point that out but I still encounter assumptions about "Western" migrants being people of European origin. Cho is American and here is what he found when he moved to Korea to teach.
Old colonial or new cosmopolitan? Changing white identities in the Hong Kong police, Social Politics (2010) by P. Leonard. How do "expats" react when the world around them changes?
White privilege, language capital and cultural ghettoisation: Western high-skilled migrants in Taiwan, Journal of Ethnic Studies and Migration (2011) by Pei-Chia Lan. Fascinating argument about how "Western" migrants "leverage" their cultural and linguistic capital in order to move abroad. This one is cited a lot. A must-read.
Identity, Gender and Teaching English in Japan (2016) by D. Nagatomo. Fascinating book. Nicely written. She's been in Japan since the 1970s and has done some excellent research. Great antidote to the misconception that migration (including marriage migration) in Asia is mostly male.