This is an extremely novel concept! Communicating lore entirely through letters is ingenious, and I found myself drawn into the world as I encountered more monsters and read more letters. If there is an overarching narrative communicated by this letters, I think I missed it, but I would love to see the same concept expanded into a larger story someday. Perhaps your letters and your ability to deliver them have a bearing on what happens. In addition, the point-pool combat system is a creative and interesting way to reward game knowledge and allow for mid-battle strategizing.
Ultimately, I think the game would benefit greatly from giving the player more feedback. For a good while, I wasn't quite sure what to do or how to do it, though I eventually managed to figure out how to move my character and travel around the map. Even something as simple as a particle effect when climbing difficult terrain could make a difference. Though I liked the pensive, mechanical score, I think additional audio cues would also help in this department. Even a "How to Play" on your itch.io page would be awesome--I'm worried I missed some major mechanics! Regardless, nice work!