I am using a critical realist approach for my mixed methods PhD study. Here are some resources that I have found helpful:
McEvoy, P. and D. Richards (2006). "A critical realist rationale for using a combination of quantitative and qualitative methods." Journal of Research in Nursing 11(1): 66-78.
Maxwell, J. A. (2012). A Realist Approach for Qualitative Research. Thousand Oaks, California, SAGE Publications Inc.
Maxwell, J. A. (2012). A Realist Approach for Qualitative Research. Thousand Oaks, California, SAGE Publications Inc.