I've written 10 books and have self-published. You're right, marketing is how you get sales but too many don't understand how to do that.

My suggestion is to break your book apart to answer specific questions on Quora (with link to your work). After you have proved teh value of the book, they will be intrigued to see if your book answers more of their questions.

The second part is to keep writing on topics directly related to or adjcanet to your book. This is like Costco giving away free samples to entice a sale.