It is related to the false document form, but more probably draws inspiration from the letters in the New Testament. It was a form most popular in the 18th century in the works of such authors as Samuel Richardson. In France, Laclos' Les Liaisons Dangereuses used the epistolary form to great dramatic effect, because the sequence of events was not always related directly or explicitly. The epistolary novel slowly fell out of use in the 19th century, especially as Jane Austen popularized techniques of the omniscient narrator.
Has nothing to do with epistemology.