I only saw the GALAXY video and my immediate thought was that I wouldn’t want to scribble over the levels. I understand that’s kind of the point but I’m confident I’d enjoy replaying the same level to “speed run” it or giving the books away to someone else later. What I think could work is a detachable acetate sheet to place over pages. This way you can play it over and over.
I would ask that you make it obvious somewhere the places you ship from. Reason being I’m in the EU and having anything shipped from the US always bites me, to the point I just refuse to do it now because it’s never worth the cost and stress. Things often get stuck in customs, and if I can get them unstuck at all I need to pay insane taxes. Being able to verify the shipping origin would help.
One game that could be fun is something inspired by ChuChu Rocket. I remember as a kid liking to solve the puzzles, and once I imagined drawing the harder ones on graph paper so I could solve them on the go.