Pretty great, love the music, went back and found the secret book after seeing a 6/7
I do think it's missing -something- to be really fantastic in the future. The puzzles are at a pretty perfect level to keep non-puzzle players engaged and not frustrated, imo. However they're not super unique. This seems like a game people would play and enjoy, but not something they'd seek out, or tell their friends about. I really like the foundation, the atmosphere, the writing, the feel and polish. But I don't think simply adding more mechanics to the puzzles would be enough to push it into 9-10/10. I was reminded of Tower Of Heaven, a pretty similar game in scope, but it stuck with me by having really impactful, memorable moments, and a sense of "beauty". Some of your non-puzzle rooms were approaching that atmosphere for me.
Maybe try to think of some other hook, memorable moments, emotion, subversion of expectations, etc. to really knock this game out of the park.
Looking at that screenshot of "The Creature's Lair" gave me a vision of a moment where you unexpectedly gain the ability to explore the areas in the water you've been seeing, maybe with like, a boat that gets left behind when you touch land, creating larger puzzles spanning multiple rooms you thought you had completed. I dunno, something cool like that where your understanding of what's contained in the game is expanded. (Maybe I'm dumb and you already planned to introduce "Surf" and "Strength" to the game at some point)
The game was good enough to make me think (and rant, sorry) about what it could be in the future, so good job!