Originally posted to rec.arts.sf.written 5/27/01, and resurrected now because it's a topic of perennial interest:
I can't produce solid figures to prove it, but I can tell you sexy babes do sell books. I've seen it happen -- author X's book XY has a sexy babe, and sells well. Next book, XZ, has a murky scene on the cover, and sells poorly.
In my (admittedly biased, and dependent on my fallible memory) experience, the following things sell books by being in the cover art:
- dragons (unicorns used to be here, too, but aren't now)
- spaceships (in a small way)
- spaceships blowing up or exchanging fire (more so)
- sexy women (more than anything I can think of)
- sexy men (slightly in SF/F, hugely in romance)
And I'm sure there are others I'm forgetting. It helps a great deal (mostly for sales of the next book) if the book actually has the cover elements in the text, but it's not absolutely necessary.
If there's a book out there that can be honestly sold by a cover of a mostly-naked woman riding a dragon while a spaceship explodes in the background, I'd love to see it. (I bet I could sell a whole lot of them.)
There are other things that help sell books -- titles are also important, and a book can't beat having been written by someone famous -- but covers are important. And the main job of a cover is to catch the eye of a reader -- not an editor, not a reviewer or critic, not a librarian or bookstore clerk, not even a relatively sophisticated fan, but a garden-variety browser who doesn't have a list of books he's looking for or a catalog of authors in his head. I said this a few days ago in the other cover art thread, but we all here are much more sophisticated and self-conscious about our reading than the proverbial "guy looking to spend his beer money."