But the real engine that sells any book after that initial push to give it a chance is word of mouth. It's you the reader recommending the book to a friend. It's you being enthusiastic about it on forums. It's you leaving a review or two.
And here's another thing you can do if you're on Goodreads. You can click the recommend button. A small thing but also a really helpful thing.
Go on, give it a go!