1. Paid targeted traffic to a sales page.
2. Free giveaways of one book, hoping to get paid sales of another.
3. Podcast/radio show guest appearances discussing the book's topic.
4. Get an authority talking about your book.
5. Affiliate marketing--get one of your books included as content or bonus in a marketer's solution package, and draw prospective customers back to you through links embedded in the book.
6. Post on relevant forums, blogs, Facebook/LI groups, Quora with appropriate feedback, linking back to your sales page/Amazon page.
7. Book a call with me to flesh out these ideas in detail.