I remember reading this book in middle or high school. It was a fantasy world, but the characters slowly realize they're in a book/a strange world because while the book is closed they can do whatever they want (I think?) but whenever the book is opened they are transported to that part of the book and must perform that page in the story. When the book is open, they can sometimes see the shadow of a face (the reader) beyond the clouds.
Does anyone know this book or this author?
by gas-x-and-a-cuppa