I agree with the author’s note that a full-on epistolary novel has to be superlatively well in order to work. But in any novel, as it grows and develops its plot, there’s room for various modes of narration. Changing the way the story is told can give the reader a different perspective on what’s happening. The pacing can shift, speed up or slow down as needed. Characters show new or different sides of themselves.

In the case of a letter, we not only get the point of view of the person who writes it; we also get a sense of their relationship with the recipient. They’ll adapt their language and slant their story in particular ways. Some of those will be evident in the letter, and others will come clear in what happens before and after it.

There are some lovely things happening in this example. The narrator shows his biases freely as he describes these simple, foolish people who have put themselves through so much horror out of fear of an angry ghost. It takes a brash outsider and a spate of mockery to save them. The final paragraph has a wry and all too topical wit that made me snort as I read it.

It’s not clear to me how the letter is relevant to the rest of the novel, but that’s not its job. The scenes that lead up to it would presumably answer that question. What it’s doing for this reader is showing the shape of a world.

Here I learn something about the geography of this part of it. I’m shown the nature of its gods and some of its mortal inhabitants. It’s evident that people come in to this world from others, and that it seems to be a regular thing, of no great concern to the travelers who come and (maybe) go.

Since this is effectively a short story, I would suggest that it be revised as if it were one, with close attention paid to the prose. That primarily means making sure every word is used appropriately and means what the author intends it to mean. It also means taking a hard look at sentence structure. Smooth out awkward constructions and make sure the meaning of each phrase and sentence is clear.

The shorter the story, the more important it is to choose each individual word with care. It’s even more important here. The letter is written in high court style, and the character who writes it is showing us what as well as who he is. If he is misusing words or phrases because he’s prone to malapropisms, that should be made clear. Otherwise, the author needs to reconsider some of the world choices.

Here for example:

I… pray you find not this volume overbearing. 

I’m not sure exactly what he’s trying to say. By “volume” does he mean this letter? And is “overbearing” meant to be a synonym for “overweening” or “overstepping”?

Or here:

menacing them outside their jambs

Does “jambs” refer to their doors? Is there a particular reason to use this somewhat odd part for the whole?

And here:

I observed Aegulf laugh into this howling abandon

“Observed” seems to be an attempt at high-styling the simple “saw”, and the ghost is howling with abandon, but the word here doesn’t quite fit its context. It might be more effective to simplify it: “Aegulf laughed amid the howling,” or something similar. High style still, but more clarity.

Be particularly careful with the meanings of words.

In thaumaturges who toiled…to molest one of Ceald, “molest” isn’t quite the right word. It’s not sexual molestation that they seem to be doing. It’s more torture or torment, or compulsion with a strong undertone of non-consent.

The husband who wanders into the woods and vanishes is a tragic figure. He’s described as doing so solemnly, which carries the implication of formality, dignity, seriousness, sincerity. It doesn’t convey the intensity of his feelings, the depth of grief that drives him to apparent suicide.

Many had perished from hunger for as we would learn later, they had consisted thus for several months. 

The dictionary meaning of “consisted” is “composed or made up of.” The word that’s needed here is perhaps “existed.”

Watch too for awkward phrasing.

Your humble servant this author, having since been acquainted with Aegulf that very morning might be tightened and clarified into “Your humble servant having met Aegulf that very morning.”

High style doesn’t necessarily mean extra words or expanded phrases. It can be concise. Sometimes it can open up a little more, for a smoother narrative flow:

He is not handsome, but presents a general warmth that is attractive could be smoothed into something like, “He is not handsome, but projects a warmth of spirit that renders him more than usually attractive.”

And sometimes it can shift from passive phrasing to active:

In your desire for comfort and safety you gave up the one thing that might save you from this horror, for there was only to tolerate some screeching for a time, the final phrase might be revised into something like “for all you had to do was suffer through an interval of harmless screeching.”

This is a nice snippet, quite revealing of the people in it and the world they live in. With smoother phrasing and more precise choice of words, it will work even better.