This was so cute. I particularly loved the contrast between the visuals and the tone. The chibi Koakuma walking around while eerie ambience and sfx play is just so charming.
I noticed that I think walking diagonally is faster? Might just be in my head, but it's a thing that happens with this kind of movement setup. Diagonal movement ends up faster than just moving left/right or up/down because of math reasons, and you have to do some math to take that into account before applying the movement inputs to the player.
Increasing spawn speed as books were collected was cool. I also really liked the idea of being able to get more books at once at the cost of speed. Though, outside of reducing the time in which you could potentially lose, there isn't technically any reason to get multiple books at once before going to put them in the shelf. Which is to say, most of the time I just got one book at a time. So perhaps an additional motivator, such as a timer, might've been good to make the player feel like they have to engage more with the speed-reduction mechanic. Or maybe more instances of having multiple books in one spot.