Amazon Ads

The ebooks since they're so high margin tend to be where most of the action is for ads. People who buy ads for their ebooks often will see sales go up for paper books and audiobooks because Amazon sometimes will default the results based off of what that person has purchased in the past. If you haven't tried Amazon ads, you really should give it a try. There are indie authors who use this one tactic to stay at the top of the charts.