If a letter arrives in a personal way, and is a fictional comic story of you and the sender, would you consider it a letter or a book?
The Lord Humungus.
Is it book sized? Are you one of those teens who call all forms of writing "books" no matter how small?
It's a letter, or, if you prefer, a letter containing a story. It isn't a book
If it tells a tale and has a beginning a middle and and an end, it is a story. The story is either in the form of a cartoon or comic strip, part of the contents of the letter from your friend, or it is folded or stapled or glued to resemble a booklet or book.
It's a short story. I did that with two fellow writers, we wrote ourselves into stories and sent multiple letters over six months doing the entire story.
It sounds like a letter containing a story.
If I hadn't asked for it I would consider it stalking. If I had asked, then it's correspondence.
A combination of both.