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.
Bram Stoker's Dracula (1897) uses not only letters, but dictation tapes and newspaper accounts.
In the late 20th century, Emma Bull and Steven Brust's Freedom and Necessity combined letters with diary entries, as did Alice Walker's The Color Purple.
See also: literature, false document.
Has nothing to do with epistemology.