From the January 3, 2019 article "READ HARDER 2019: AN EPISTOLARY NOVEL OR COLLECTION OF LETTERS" by Annika Barranti Klein.
"Epistolary
"adjective
"(of a literary work) in the form of letters.
"'An epistolary novel is a novel written as a series of documents. The usual form is letters, although diary entries, newspaper clippings and other documents are sometimes used. Recently, electronic "documents" such as recordings and radio, blogs, and e-mails have also come into use. The word epistolary is derived from Latin from the Greek word ἐπιστολή epistolē, meaning a letter (see epistle).' —Wikipedia
"I jumped at the chance to write a book list for this Read Harder task, because I love epistolary novels and I love reading letters between real people! I've grouped suggestions by type of collection, with fiction down at the bottom. Feel free to skip down, but you may be surprised by how lovely it is to read a real person's letters."