The book as a playing field works really well. It kept me invested in the story all the way through. Sprite work is charming and, more importantly, consistent enough to not distract from the narration as a whole.
Music choice is kind of a mixed bag. The happy intro theme kind of clashes with the melancholy that follows during gameplay. From a story perspective, the dark jingle bells is quite functional, but on the other hand it lacks the tempo and drive that is common for the genre (Binding of Isaac is an example where I feel the music enhances tempo of gameplay).
Audio is functional and non distractive. Going into nitpicking, I would suggest adding a random pitch and/or gain to the attack sound - it would perhaps make the sound more dynamic?
Balance wise I feel the game treated me fairly all the way through. Perhaps too fairly, as the abundance of player "health" had me breeze through the game the first time around. It would have been interesting to see more punishing enemies in the later stages, perhaps coupled with some kind of power up, forcing the player to adapt and learn new things on the go?
All in all, I feel that the game was well executed. Good work!