Fascinatingly broad question, and not really my cup of tea, so I'll give some works I have and try to link them to your interest. I'm interested in the Analytical tradition, so this question seems more bent to the Continental side of philosophy. I've never really thought about what traditions there are in sociology so bear with me. I would say approaches in social relations include phenomenological and existential, structural, critical theoretical (Frankfurt), Chicago social, and others listed in detail below.
Sociology is a broad topic that includes institutions of journalism, science, law, education, and politics, so what you have is a tall ask. If I had to say for foundations from my own paper library, political, legal, educational, and economic science before or during the 19th century, some important works for social relations broadly are:
You're probably familiar with those works, but maybe you've missed one or two.
From the European traditions, phenomenology and existentialism are very individual-centric but can touch upon social themes. Derrida is known for his deconstruction which is a very continental approach to language, which is in my opinion the most important social enterprise. (I'll touch on that later.)
I would suggest critical theory is a major school on social relations which was articulated by the Frankfurt School and is very much in opposition to the logical positivists who favored empiricism very heavily. The only two I've read anything of are Habermas and Adorno. I've read The Structural Transformation of the Public Sphere and some educational essays tied to it when I was a teacher, but I do have The Theory of Communicative Action queued up and it looks promising.
From the Continental movement of Structuralism, the only one I'm familiar with is Saussure because of my interest in semiotics and philosophy of language. I know Levi-Strauss is a staple for anthropologists, but I've only read excerpts.
There are a number of Europeans on this site who have more insight.
From my education days, I've come across Auguste Comte who is famous for his positivism, Ronald Dworkin who is famous for his legal positivism; I've got A Theory of Justice by Rawls sitting around unread, and from my hometown, works by George Herbert Mead and John Dewey whose books were required reading in classes I took at the University of Illinois. They were part of the Chicago school and are part of the powerhouse of education and economic intellectuals which regrettably I don't have much of a taste for. In the Marx tradition Pedagogy of the Oppressed was big with educators and that got me reading about but not primary works of liberation theology which looks at the intersectionality of religion and socio-economic status and was big in South America in the '70's. Friere and others are part of an educational philosophy known as social reconstructionism or critical pedagogy which includes Friere, George Counts, and Theodore Brameld. Never read anything by the last two. I liked whatever I read about Piaget and Vygotsky, selections and summaries mostly. That seems to do it for education. There are other learning theory and philosophy of education works in learning theory like Allan Bloom, John Holt, and Bruner who certainly aren't giants in philosophy but were champions of progressive education.
I would also suggest that there are two other approaches, which are philosophy of science-related and sociobiological-psychological for lack of a term.
If one wants to understand the biopsychosocial basis for human relations, I'd suggest reading On Human Nature and The Social Conquest of Earth both of which purport to show the evolutionary origins of human relations. I love everything I have by E.O. Wilson. An ethologist who works in the philosophy of the biological foundations of social relationships would be Frans de Waal. He takes a philosophical look at primates' eusocial behavior and ties it to political science and morality. I love his Chimpanzee
Politics and The Bonobo and the Athiest. Science since Darwin has made tremendous strides in showing how much intentionality animals manifest and making Descartes and his presumption about animals being automatons. If you're touching on the philosophy of mind there seems to be a deep undercurrent of solipsism woven into philosophy before Gilbert Ryle and W.V.O. Quine.
And speaking of social relations, one would be remiss to talk about sociolinguistics and John Searle who is considered a social philosopher as well as philosopher of language. I'm familiar with his works from the linguistic side because of my interest in speech acts. The four books that Searle has written are Speech Acts, Intentionality, The Construction of Social Reality, and I started reading Making the Social World. Late Ludwig Wittgenstein and his Philosophical Investigations is highly influential because of his connection of logic, language, and intentionality and his discussion of language games and public and private languages.
My interests in cognitive science have me invested heavily in the intersection of the biological and cognitive linguistical basis of mind and society.
Oh, and as far as the intersection of sociology and the philosophy of science, of course the most famous would be Kuhn and his The Structure of Scientific Revolutions. He exposed the lack of objectivity in science that the logical positivists tried to prove existed. Blackwell's Companion to the Philosophy of Science has two entries The Social Factors in Science and Social Science, Philosophy of, and I'll mention some names that are pretty common to see in the entries: Hacking, Latour, and Fuller. It's about time I got some of their work on social epistemology.
And I read The Social Construction of Reality by Berger and Luckmann which claims to draw heavily from Max Scheler and is considered important in the sociology of knowledge.
Anyway, I hope something in this pile of words is of help. Good luck, and thanks for the interesting question!