An epistolary novel is one that is made up of a collection of letters, articles, or journal entries. Mariama Ba’s So Long a Letter is an epistolary novel because, as the title suggests, the novel is a long letter. So Long a Letter tells about Ramatoulaye, a widow living in Senegal, who must not only grieve her husband’s death but also understand how her husband cold have participated in a polygamous marriage when he knew she would not be accepting of it.

In the novel, Ramatoulaye writes a letter to her childhood friend Aissatou. Over the course of the letters, the reader is able to join Ramatoulaye on her journey of self-discovery as she mourns the recent death of her husband, Modu, and questions their relationship’s foundation and his participation in a polygamous marriage.

Because Ba structures her novel as an intimate letter, the reader is able to gain backstory on their relationship through Ramatoulaye’s eyes and feel her pain as she questions how her husband could marry their daughter’s best friend. The novel’s structure also provides Mariama Ba with a medium to question Senegal’s acceptance of polygamy in a modern society. Instead of just writing an argument against the practice, Ba has chosen the epistolary genre to provide intimacy and connection with her main character, with the intention of swaying her audience to see her point of view.

One of the novel’s main themes centers on modernity versus traditionalism. As Senegal becomes a more modern nation, Ba argues that they must leave some old traditions in the past. She argues that the nation cannot be a modern Muslim nation and also embrace polygamy.

I think you mean "epistolary." The word "epistle" comes from the Greek word for "letter." So an epistle is a letter. Epistolary means "in the form of a letter." An epistolary novel would be written as if it were letters to and/or from someone.