I'm in the process of writing a paper for my MA in Japanese Studies, and have been reading some books that I would like to recommend.
1. Miller, Roy Andrew. (1982) Japan's Modern Myth: The language and beyond
How Japan's language and racial identity are linked. Very good social commentary.
2. Maher, John C. and Yasuhiro, Kyoko. (1995) Multilingual Japan. Multilingual Matters LTD.
Eight informative articles on the multilingual nature of Japan and its various linguistic minorities.
3. Denoon, Hudson et. al. (2001) Multicultural Japan: Palaeolithic to Postmodern. Cambridge University Press
16 essays on multicultural aspects of Japan and Japanese culture. Very comprehensive.