"Epistolary" means it was written as if it were a letter or email. I think you mean "expository." An epistolary exposition could easily be a thing.
There's lot of expository writing in non-fic, and much of it anything but bores the reader it done well. Writing fiction in this style can be quite effective. What makes it done well? For me, it's if I'm learning something. If it builds then unravels an intellectual mystery for me. There doesn't have to be a big "tah dah!" reveal at the end, but I need to be more educated on a subject I care about. The tension, maybe, is between me and the subject. If the author can anticipate what I already know, play on what I don't know but want to, then lead me through the exposition, I am hooked. Even if it's a fictional topic or event.
Good non-fic examples: "How I Became a Prison Gardener" by Michele Scott. This originally appeared in The Marshall Project and is included in the 2016 Best American Non-Required Reading. Why it worked for me is that prison life is somewhat mysterious to me. All I have are stories from people I know who've been in, friends who've done prison ministries, and (the worst source) TV and movies. Another, also in the 2016 BANRR: "Death Qualified" by Gary Indiana (shoot, either that's a genius pseudonym or he has very cruel parents), which is a book review of a biography of the Tsarnaev brothers (the Boston Marathon bombers). Why it worked for me: It educated me on the subject without me having to read the entire book Indiana is reviewing. A podcast that sets the standard for me is Radiolab's
HIV Patient Zero: The Origin of AIDS. Give these a read/listen if you can find them. I'd eagerly read fiction written in this style.
An example that doesn't work for me:
The World Until Yesterday by Jared Diamond. This is the fourth or fifth book by Diamond that I have read, so to me it's very predictable. It's a long list of generalizations based on four cultures Diamond knows a lot about, and his conclusions are pretty much "well, duh," even if I wouldn't have thought about it before. There's no unraveling of a mystery.