I really liked this! Very fun, and I also love language puzzlers (Recommend checking out Heaven's Vault if you haven't!)
Currently ended up at 98% complete, struggling with the secret translation and one of the concepts, but I'll see if I can have any ideasa tomorrow.
My main nitpick in terms of user experience would be that when I click on a word to to type it in, I am no longer able to change pages, I have to close the book entirely and re-open it. Also being able to use the scroll wheel to navigate pages might be a good QoL feature.
I think this could also be very fun to see expanded if that is something that interests you.