A lot of times, I see authors talk about promotion. It's something we have to do, whether or not we like it. The earlier you accept this, the better mindset you'll have. I see lists of ways authors can promo their new releases, or even themselves (branding). And almost everything I hear is spot on. But one thing I rarely hear, and what I feel is the most important thing, is backlist.
Best thing you can ever do for yourself as an author after your first book releases is...release another book. Let's face it, it's free promo for another book to release. All you had to do was write the book. You didn't have to send out anything to reviewers, you didn't have to pay any money to get your cover on a site or in a newsletter.
Why is another book release promo for an previous title? Say someone picks up your newest book. They love it. As in they cherish it because it was so well written, and kept them on the edge of their seat every single page. That reader is now apt to do a little search and see if you have any other books they can dig into...and love the same. *BAM* Sale of one of your older titles.
And you didn't do anything to promote that title.
So, I repeat (in bold): Best thing you can ever do for yourself as an author after your first book releases is...release another book.