A literary work written in from of a letter is

A literary work written in the form of a letter is called an epistolary work. This type of literature is structured as a series of letters exchanged between characters or as a single letter written by a character to another character or an intended audience. The letters may be real or fictional, and they reveal the characters' thoughts, feelings, and motivations. Examples of epistolary works include "The Color Purple" by Alice Walker, "Dracula" by Bram Stoker, and "Les Liaisons Dangereuses" by Pierre Choderlos de Laclos.