The books are informative but not essential. One is "The Mystical Weapons of Arturus" (which you get to do battle) and the other is Mordred's Journal, which is missing an important page. (see related question)
There are two books in the library and one in Mordred's laboratory.
In the library.
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.
It's downstairs. Press one of the stacked books with a paper on it, and it should get you the library card.
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.
you click on the books next to the bed
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.
There are three informative, but not crucial, books in Arturus. Read them if you wish: # Library : "The Mystical Weapons of Arturus" # Library : "The Life of Mordred : A Cautionary Tale" # Secret Lab : Mordred's Journal
Use the gold coin from the fountain to get the library slip from the books at the Mordred Museum. Go to the lower aisle of the castle library, and find the shelf marked McM.
Click on the stack of books next to the man and get the Library Slip. You can use this in the castle library, on the lower aisle where the shelf is marked M-Mc.