The colors of the UI blend really well with the overall tone of the backgrounds! Everything has this aged paper-like texture, and I think you've done a great job of maintaining that distinctive aesthetic throughout the game.
(And just as a side note, I absolutely love the book's design, especially the cover! I kind of wish I could have a copy of it myself.)
That said, I feel like the character sprites look a little disconnected from the backgrounds. If it were me, I'd probably try to make the overall color palette a bit more consistent to help everything blend together.
Also, I found a bug! I'll attach a screenshot to show you.
