Hey.
First of all, 'Metroidvania Book Game' is a pretty good hook in my opinion. I don't know about the name 'Cosmogonic Overflow' though, although I'm sure you picked it for a very specific reason.
The setting, and the atmosphere is really cool. It's like a weird sci-fy, lovecraftian thingie, and I was pretty immersed the whole way through. I especially got excited by when things got a bit meta. So, the thing were you grab a page and paste it. Or, the elevator page that has holes in it. Or the fact that you can read things across the page. That shit is cool, it ties really well with everything else, and makes me wonder what other things the game will do next.
Hated the combat, couldn't figure it out, reading the instructions pissed me off because I couldn't understand them. I ended up spamming auto-battle.
I was usually confused when backtracking. I found myself clicking on things sort of wishing they would led me to where I wanted to go. I got disoriented pretty quickly. It didn't cause me issues to progress forward, however, I guess.
The other thing is, sometimes I feared forgetting something, and would note it down on a physical paper. I wish there was a way to do that in-game.
A couple of little improvements:
* When you get mind scrambled by the last enemy, it would be nice if you explicitly hinted in bold that the keywords got changed, because I thought it was a bug when my keywords didn't work
* It'd be nice if you didn't have to open the menu to access the 'Password' button, I wish it was just out in the open, since it's such an integral part of traversing through the book
* I didn't notice the 'Paste' button at first when pasting the cut page, and wandered around confused for some time. It'd be nice if it was more visible somehow. I also tried to drag the page into the book, which it would have been nice if it'd worked.
Presentation wise, what you have is pretty good. But I'm going to be overly critical just because your concept is so good. So, first, the drawings and the 3D effect on the book is perfect. The pages themselves feel a bit flat, like it's just a gradient, they remind me more of a webpage, rather than a book page, if that makes sense. In that same vein, the links look literally like web hyperlinks. Again, nitpick, but I feel like that's like slightly off the setting. And finally, the turning page animation is a bit too simple. I also wish that you could see many pages turning when you skip pages. Some VFX to juice it up and make everything more creepy and unsettling would be nice I think, although I don't have anything particular in mind. And lastly, just a thought, but it would be nice if the setting sort of creeped through the page. So, in an area that has fungus, maybe the page gets moldy. That'd be cool. All in all, again, your presentation is cool, and it doesn't deter from the experience, but if you really juice it up it could be something really amazing, I think.
Uhhh... So yeah, I think your execution of the concept is pretty godlike. The demo definitely leaves me with very big expectations. If you ever release a full version I'd be stocked to play this. Very, very cool bro.
Have a video in case you find it useful: