Hi There! Really loved the game, its super fun!
In my humble opinion there are 2 things that bothers me.
First is the hitbox on the enemies, i feel like sometimes i try to get away from getting damaged in the enemy swarm, but i still get hitted. Maybe giving a little more space to move around so it can have a little bit more of skill progression would be nice (imo at least).
On the same topic, another thing that could be improved is a tiny invulnerability frame when you get hitted, so you can try and get to a safe place if you are hitted.
My second point is that it would be awesome that if the book continued to stay on an enemy sprite it would still damage it, in this way it opens up for a reward in skill if you are pacing around an enemy and are able to do this.
Well, this are little things that i feel could improve the gameplay a lot. I really loved it!