Variants allow you to define specific options for your book. For example, you can create a Product with a 6x9 hardcover option, a 6x9 paperback option, and an 8.5x11 coil bound option, and all will be listed in a drop-down menu from the individual Product's listing page.

When creating a Shopify Product, you will also be able to create a Product Variant for your book. Please see the following article from Shopify for more information:

Once the Variants are prepared, you'll be able to assign each a specific file for the interior and cover to match the Variant options using Lulu Direct.

To connect your interior and cover files to your Product, go to Apps > Lulu Direct > Books, then locate the Product you just created on this page. You'll see all the Product's Variants listed here. Click on the Variant you want to connect, then click "Update" and you'll be guided through the file upload process. 

Once your files are uploaded and approved, click "Save" and the book will be ready to list in your Shopify store. 

According to Shopify documentation, a Product can have up to 100 Variants.