Creswell, J and Plano Clark, V. (2011) Designing and Conducting Mixed Methods Research. 2nd edn. London: Sage Publications.

Chapter 1 will probably tell you as much as you want to read about mixed methods. There is a long discussion of all the different definitions of mixed methods.

Yang, K. (2010) Making Sense of Statistical Methods in Social Research. London: Sage Publications.

Have a read of Chapter 2 which considers what (variable-based) statistics are good/not good at. Also have a look at his ten rules of using statistics.