The competition is fierce, both for fiction and nonfiction — especially with the new ebook and print-on-demand technologies allowing anyone to publish. But, as you noted, most sell few books.
To sell more books, you really have to establish relationships with websites devoted to the genre of your novel (or the subject of your nonfiction book). Get interviewed by them. Offer them excerpts from your book. Offer free ebook copies of your novel to a limited number of their readers. Offer them a review copy.