Fantasy Children's Book; Characters Must Race Into Place to Play Roles on Each Page

Question

• Characters are physically trapped inside of the book you are reading, they have to run around inside of the book to make sure that the story moves forward. They are their roles, but also self-aware that their kingdom is in a book.
• Possibly titled "The Great" something
• Female main character, possibly a princess?
• Hardcover had a square shape instead of the traditional rectangle
• Pretty short, under 200 pages

Answer 1

Continued Googling brought results.

The book is The Great Good Thing by Roderick Townley.