If you are talking about the books in the Museum of Mordred, they are at the top next to the bed. When you click on them, the museum guy will tell you that the books have to stay where they are, but that you can have an old library slip of Mordred's.
There is a gold coin in the fountain outside. Go in and get the Library Slip from the stack of books.
You go to Mordred's Museum and click the books on the top floor. The manager won't let you have the books, but you get the card.
In the library.
McM
The Library Slip is among a pile of books at the Mordred Museum. Get the gold coin from the fountain outside to pay your admission.
There are two books in the library and one in Mordred's laboratory.
Use the gold coin from the fountain outside to pay your admission. There is a library slip in one of the books, indicating a particular location in the castle library.
Go back and get it. Take the gold coin from the fountain, and pay your admission to the museum. In the stack of books to the right is the Library Slip.
Go in the castle and go on the door nearest to you on the first floor. That is the library. Look around and you will easily find the books.
you click on the books next to the bed
You use the gold coin from the fountain to enter the Mordred Museum. Pay your admission and find the Library Slip in the stack of books. This will access the dungeon in the castle library.
In the lower aisle of the castle library, in the Non-Fiction section. You cannot use the secret passageway until you have the Library Slip from books at the Mordred Museum. Get the gold coin from the fountain and find the slip in the museum.