For background, I'm working on my master's dissertation for a data science project regarding COVID and want to reference some background information about health disparities. I've done some academic writing before but mostly for classes which aren't anywhere near as demanding in terms of research and citation accuracy. I've been using the citation lists in existing articles to generate new sources to read since I know that in humanities, it's important to cite primary sources if possible (and I am assuming citing the source of a study tends to be better than citing a literature review, for example). However, one of the books I'm reading is somewhat tangential to my topic and still has studies referenced within that (according to the rule I set for myself) I would need to cite as well.
Is there a rule of thumb for when you can "cut off" your research notes and avoid adding sources that barely relate to your subject?