Keeps track of books read by your Neopet and removes their name from the dropdown list in inventory for reading a book if they have already read it. On the home page (https://www.neopets.com/home/index.phtml), it will remove books from the list of options when you select "Read" on a pet, and re-calculate the actual number of unread books you have in your inventory and available to read to them.
@Julia Huth, when I wrote home page, I meant https://www.neopets.com/home/index.phtml -> the page login redirects you to.