Answer: I have no idea, leave it to the professionals.
Ok, now that the joking answer is out of the way…
Once upon a time yours truly worked at a bookstore. This was many, many moons ago (back when years started with 1-9 instead of 2-0). And I still to this day remember the cover of one of the books that came out during that time, because I was able to use that book cover to sell copies of a book that otherwise had no reason for selling as well as it did. (And I mean no offense to the book, I assure you.)
So, I have no idea what makes a good book cover, but I will say that this was the best book cover I have ever seen.
The book in question was called Eat Me by Linda Jaivin. (I still have a copy of it, because even I bought a copy for the cover.) It’s an erotic novel where four women discuss their sexual fantasies. I remember something about a creative use of figs and a truck-driver, but other than that, the book didn’t really make any lasting impression on me.
But the cover…ah, the cover. Was perfect for what it needed to do, which was catch a prospective reader’s eye. And a certain type of prospective reader.
The book had two possible covers. One involved a banana and two strategically positioned plums on a bright green background. The other consisted of a papaya* with the inside partially exposed on a bright red background. Whatever was on the front cover, the back cover would be the opposite image, so each book had both images. (Unfortunately, the on-line photos do not do this cover justice.)
So, anyway. I would go into work and I would find our copies of this book and I would place them face-up or face-out on the hardcover book display table. And, specifically, I would make sure the “female” image was on display even if it was the back of the book. And when I did that, those puppies sold like hotcakes.** Literally, if someone had turned the book so that only the spine was showing, no sales. The minute I turned it so that cover was visible, sales.
I worked at a small independent bookstore in a posh area. No one came in asking for our erotica section. No one ever asked for that book by name. But, on the power of the cover alone, I was able to sell 20+ copies of that book at our little store.
What made it so effective? It was simple, it was eye-catching, it was titillating in a sophisticated sort of way, the colors made it stand out from all of the books around it… something about that cover appealed to people. I don’t think it would’ve been effective for a non-erotic title and it wouldn’t have worked if every book had a similar color palette, but it set itself apart in the right kind of way.
Interestingly, I don’t think the folks who try to sell this book realize the power of that cover, because when I did an internet search for the book most of the images were of the “male” cover. And the author’s website had another cover that has the “female” image, but an artistically drawn version that is lacking the impact of the version I linked to above.
So, a conclusion: book covers can be incredibly important for driving sales. Incredibly important. (Just remember that “you can’t eat the pretty,” so if you want people to buy your next book, too, you better back that pretty cover up with engaging writing as well.)
* I think it was a papaya – I’m not good at my exotic fruits
** Sorry, my descriptive abilities are a bit lacking today.