1. What is purpose of letting customers create accounts?
If you have club prices set on your website, you will be able to give allows visitors and customers to sign up for your websites newsletter. You can offer them a special price for being members (club price) if you want to encourage them to join. It's good to have a big list of customers and club members so you can market to them when you want to drive sales.
2. What is the “Join our Book Club” page?
This is a marketing angle to encourage your customers to create an account on your website. You are not required to give club discounts do any additional activities to maintain a "club." However you could offer club promotions or send quarterly/monthly newsletters to keep customers remembering your website.