What sells a book?

I guess that should really read "who", not what, because what sells a book is the author.
Not the publishing company. If you have written a book that you believe in, you have to be the one to get out in front of people.
I hear the groans already. "I wrote it, why do I have to sell it, too? Isn't that the job of the publisher?"
Yes, and no.
The publisher is running a business. They are going to put their money behind a book they know is going to give them a return on their investment. So, sadly, they are going to spend money on the politicians, the controversials, the known names, because the public will flock to them.
Unless you are a household name, you are going to have to make sure your book gets some press.
And I know that's hard. I have to admit, I would probably prefer dental work to getting up there and making a presentation. Oh, I do all right once I get started but the anticipation kills me.
Yet, it's a necessary evil.
Don't believe me? Check here and here and here. There are dozens and dozens of other articles out there that can attest to this.
So, write the best book you can. Then market to your best ability. Use Facebook and Twitter. Write blogs. Network. And know that it still might not work. The competition for your book's audience is fierce. But at least you will know you gave your book the best chance it had to making it. And that's all anyone can ask.