I recently enjoyed Language and Social Context: Selected Readings (Penguin, 1972), edited by Pier Paolo Giglioli. It includes some articles so famous that even a non-linguist like me knew them (John Searle on speech acts, William Labov on nonstandard English), along with many that I didn’t.
One article I especially liked is ‘The Speech Community’ (1968) by linguistic anthropologist John J. Gumperz, in which he describes that unit as ‘any human aggregate characterized by regular and frequent interaction by means of a shared body of verbal signs and set off from similar aggregates by significant differences in language usage’.
The idea of a speech community is very useful when discussing and thinking about language: we easily forget how highly social and context-dependent are the linguistic rules and norms we observe more or less unconsciously. Gumperz goes on:
Most groups of any permanence, be they small bands bounded by face-to-face contact, modern nations divisible into smaller subregions, or even occupational associations or neighborhood gangs, may be treated as speech communities, provided they show linguistic peculiarities that warrant special study. The verbal behavior of such groups always constitutes a system.
There is coverage of speech-community boundaries, cross-cultural influence, dialect variation, language loyalty, and more. I can’t say how the essay may have dated in the half-century since its first publication, but it seems a clear, accessible introduction. You can read it all here (PDF), or (maybe) access the full book at the Internet Archive.