I like that there are a lot of things that need to be taken into account when collecting the books like the spawn interval changes when Koakuma collects more books or the Koakuma's speed decreased with how many books she has.
The sound effect is also on point. It gives an eerie feeling but also acts as an alarm that something will be coming on.
I streamed the game here around minute 18 in case anyone is interested in watching it.
Great job with the game!