This is a very well written self-discovery story aimed at middle grade, with a haunting providing the motivation for the main character's journey. The main character figures out that

he is a trans boy, and the descriptions of his dysphoria are so close to my own experience that I guessed before reaching the author's note that Lukoff is also trans.

I did feel that the characters seemed younger than their stated age, more like 8 or 9 than 12 or 13; but since that would conveniently make them the right age for to be starting at an English middle school rather than a US one, I just visualised them as such. I will definitely look out for more of Lukoff's work.