You should refine your question or provide more context. Are you asking for reading materials for (1) doing research in some typical areas of the discipline of philosophy or a general background reading list for doing a research degree in philosophy, or (2) are you using "philosophy" to refer to some set of methodological guidelines for research in some other discipline? (If so, what kind of discipline -- humanities, social sciences, natural sciences...?)

Thank You Karl Pfeifer for the answer. Actually, I am looking answer for my thesis. I was trying to work on my MPhil thesis in populations studies but my supervisor asked me to add some research philosophy on your background of research. I got confused what to write on it !!! Hope you can again help me to work on it !!!.

Thank You!!!

I still don't understand what you mean. Could it be that there are different methodological approaches for how research in population studies might be done (e.g. probability vs. nonprobability sampling) or "schools of thought" regarding an overall ideological position (e.g. Malthusian vs. Cornucopian), and your supervisor wants you to explain and justify your general approach? What sort of "population studies" are you doing and in what discipline(s)?