Hope following suggestion will help you:
1. At Amazon Kindle
2. Facebook Ads
3. Google shopping
4. Email marketing
Nick here from Shopify. Great question!
I'm thinking the best way to sell e-books would be via a digital download app. Digital download apps basically let you sell non-physical products to your customers who would get a direct download after checkout and an emailed link. There are several different types of digital download apps in the Shopify App Store which you can look into for this. Let's take a look at some of them below:
You might also be able to develop something customized by working with a Shopify Expert, but it would a much larger upfront cost to do this.
Is this along the lines of what you were thinking?
All the best, Nick