A great starting point for background info is Gale eBooks, which contains subject-specific encyclopedias. Simple searches (2 or 3 words) work best here. (e.g. Puerto Rico culture or Ireland emigration).
Here are a few books you might find helpful, but there are many, many more.
American Immigration: an Encyclopedia of Political, Social, and Cultural Change
Encyclopedia of the African Diaspora
Encyclopedia of Immigration and Migration in the American West
Native Diasporas
Worldmark Encyclopedia of Cultures and Daily Life
Asian American Culture
Occupational Outlook Handbook
