What do you do after you get the books and the book slip in Astro-knight Island?

After obtaining the books and the book slip in Astro-Knight Island, you need to return to the library to use the book slip on the specific book it corresponds to. This will unlock a hidden passage or reveal important information needed to progress through the game. Once you've gathered the clues or items from the book, you can continue your quest to solve the mysteries of Astro-Knight Island.