Quick-take: Nice Mistborn-like story.
Lia is a wretched. She was abandoned as a baby on the steps of Muirwood Abbey. Now, a child in her young teens, all she has known is cooking and cleaning under the care of Aldermaston. Her life takes a sharp turn when a soldior lands in her care.
Lia reminds me a lot of Vin from Mistborn. A young woman of low stature but with inherent talents. I won't go in to more detail as each step of her history and abilities are key areas of the plot.
The lore of the story gets a bit too pseudo-religous. The "Medium" flows through the land and can be harnessed to do various things. It also seems to have a mind of its own. The inhabitants trust and rely on the medium in their day-to-day. It is discussed a lot in the intro-letters with a sermon vibe to it. Lia is a bit of Mary Sue as she embraces the Medium.
The relationship between Lia and the soldior is a bit odd. It goes from a bit like a fatherly figure, to older brother, and it seems to move in to love-interest terrority. I simply do not know where that relationship going, but I will take the assumption that Lia, being around 14 years old, probably has no idea either. The book is simply reflecting her own confusion. Whatever way it goes, the soldior seems like an honorable chap.
I always appreciate having this: the ending of the book was a decent ending that closes the main story arcs. There is certainly more to go, and I have already picked up the next book.