"Epistolary" means a story told through a series of documents. Letters, journals, diaries, reports, any source of information, primary or secondary, written by the "characters" rather than the "authors" (thus, making those two words very complicated).

This is for those stories that are told first-hand. No omniscient narrator knows everything here. The point of view can shift. The style of writing can shift. Even the format may not be stable, with a series of letters broken up by a portion of an essay, a page from a script, or even the transcription of a recording.

Epistolary stories present their stories straight from the source. So, consider this a source for sources.