you don't need one
The library card is in mordreds house. all you have to do is click on one of the books (as long as you have paid admission to the house) and the dude will say that nothing can be moved but you can have this old library card.
On Astro Nights Island
the leftsome answer
it does not say library card you have to click McM because that what it says on the library card again not library card
its some were in the royal library
The book is luying around the library
The McM is on the bottom floor in the library. You can click on the mcm if you have the library card. The library card is in Mordred's house museum.
In Mordred's house
In the museum of Mordred click on the books in the chair and he gives u the library slip
On your library card{If you got it} it is at the top.It is underlined in red it's in non-fiction McM.Hope that helps!
you have to get the library card from the guy in the museum go to the library in the castle click the library card look at the genre of books he checked out click the gold thing in the library that matches the card and a stairway will appear that is the dungeon.
first you get the coin then you pay the guy in the museum and click on one of the books in the museum.