Digital Products (eBooks, music, movies, what have you) are added in pretty much the exact same way as Physical Products: go to the the Products section of the Store tab and click Add Product. Then write description of this Digital Product and add as many images as needed.
Where it begins to differ is when you select your Product Type, as instead of adding a Physical Good you'll want to select a Digital Good.

Once you've made this selection you enter the price of this digital good (or goods) and upload the file(s) you want to provide to any customers who purchase this product.

A Digital Product can consists of one file or several files. It's up to you as to what you'd like to include in the price of this single purchase.

After the files are uploaded, you'll likely want to set a limit either on how long or how many times the files can be downloaded. Without a limit your customer will be able to download the file an unlimited number of times, which makes it a little too easy for them to share the link with other people.
When setting the number of days or number of downloads it's helpful to keep in mind that some customers will make a mistake when first downloading the file. Or accidentally delete the file. Or do any number of other things you might not expect. You want to give them leeway so they don't have to contact you if there's a problem.
Providing a download limit of 5 to 10 days / 5 to 10 downloads is a good rule of thumb so your customers have some room for error.

When your customer purchases a digital item, they'll be able to download that item the moment they check out since we display the link on their order confirmation page.

But if they gloss over this page, they'll receive an email that includes the download link. You'll also receive a copy of the email, giving you a back up of the link that you can provide them in case they also lose their email (that may seem unlikely, but stranger things have happened).

Other than what you've seen here, Digital Products are functionally the same as Physical Products in terms of how you sell and display them on your Storefront.