Colin Robson's Real World Research is a good methodology book.
Obviously you will consider the survey and the semi-structured interview - why you are using them, their strengths and weaknesses as methods, what kind of data you hope to get from using them.
In the 3rd edition of the book, Robson calls the mixed method approach a "multi-strategy research design", where "qualitative and quantitative approaches are combined in the same project" . Does this fit with what you are doing?
Is this a common approach in sociological research? Can you come up with some examples where this same / similar research design is used? Look at how they have justified this approach when researching social subjects.