When you want to change the books that are displaying on your storefront, you will need to start in 'Pages' because that is considered a page.
1) Click 'Pages'
2) Go to the active homepage (the homepage is the first page your customer will see when visiting your website)
3) Go to Layout/SEO tab to edit the product categories.
4) Once you click on the Layout/SEO, you will be able to manage the categories that show up on your
homepage. If you do not want to take advantage of all three category previews, just set the ones you do not want to 'None' in the drop down menu.