An epistle is a letter.
An epistolary novel can be fiction or creative nonfiction. It can tell a story through one or more letters.
(from Virtualsalt.com )
Code Name Verity by Elizabeth Wein
See more examples here: Goodread’s list of epistolary novels.
Epistle defined here: dictionary
Today’s Assignment: Write a short story in a letter format. Alternate letters between two characters.