I've been reading around the blogsphere and reading reviews for a long time and checking out Amazon and I've come to the conclusion that the only thing that really sells books are reviews.
Every time a reader takes a moment to give something two stars or five hearts or an in-depth analysis for why the story did or didn't work, the writer gains. The book prospers. The world, as we know it, is a better place.
There was a discussion on Smart Bitches about promotion for books and a I've been looking into promotion recently also... any author wants to get her work noticed. My conclusion is there is no effective promotion except readers leaving reviews on Amazon and Goodreads.
So as a small author and a big reader, I ask that if you read a book you loved, leave a comment on Amazon. If you read a book you hated, leave a comment on Amazon. If it left you in a meh state of mind, that's right, leave a comment on Amazon.
You can't do more any more effectively than doing that.