You will be able to purchase new Bookshelves for your Neopet in the Bookshelf page, and the Bookshelf Customisation widget. Each Bookshelf will cost a set amount of NP that increases incrementally by 500 NP with each one purchased. The first Bookshelf you purchase will be 3000 NP, the next will be 3500 NP, the one after that will be 4000 NP, and so on.
You can also earn extra Bookshelves for FREE by reading a lot of books to your Pet! For every 150 unique books you read to your pet, you will earn a free Bookshelf. This will automatically be granted to you, and will not count towards the number of Bookshelves you have purchased (which means that if you’re going to pay 5000 NP for your next Bookshelf, then you earn a free one, the next one you purchase will still be 5000 NP).