Online promoting your book is about getting your book out to the world in as many ways as possible and to as many people and places as possible. The more eyes you can attract to your book, the better your chances of making book sales.
Re-Post one of your 5-star book reviews
Add a comment on a popular book blog
Publish to Medium
Make a Facebook Page
Run an ads on Social Media
Place an ad on your blog
Publish Guest Article on High Traffic articles and Blog