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: