Library Book Slip
The Library Book Slip is not really a required item in Astro-Knights Island, but it gives you a clue to a hidden area. Pay attention to the underlined parts: McM.
How to Get the Library Book Slip
Go back to the first building, the House of Mordred. Talk to the man you see when you enter and get the pamphlet. Then use the coin in your backpack and he will tell you that you are free to look around. Next, click on one of the books. The guy will ask you not to touch them, but will give you a library slip.