So what separates the good covers from the bad? I think there are a couple of factors that may contribute to an effective cover: one is an implied narrative, another is layout. Cover illustrations often tell a story in their own right; preferably one that pertains to that which lies within. It isn't absolutely necessary for cover art to tell a story, but if it does it should be an honest and accurate one - some covers are disingenuous and promise one thing but deliver another. I recall being often disappointed, in my youth, by publishers in the 1970's who all too often pulled this bait-and-switch tactic to sell paperbacks. In those days all a publisher had to do was slap a Frazetta painting on a book and it would sell. I should know - I'm the one who bought them.
Contrary to the implication of this cover on the Ace edition of Edgar Rice Burrough's The Cave Girl, there are no spear-wielding, sabre-tooth-tiger-taming, fur bikini-wearing amazon women in it. It's actually the story of a gentleman who was shipwrecked on an island and who discovers a young woman who was similarly stranded as a child. They have some adventures after which he takes her back to civilization and marries her. The Frazetta cover version looks more exciting though, doesn't it?
Here's the cover of the Pinnacle edition that hits a little closer to the mark:
The cover of The Man in the Iron Mask depicted below is an example of a poor choice in cover illustration, not because the painting is bad (although it is drab and not especially eye-catching), but because it has absolutely nothing to do with the story. Philippe was incarcerated in the Bastille, not the crumbling ruins of a tower in the remote countryside. I have a feeling that this illustration might have been stock art that Signet had on file, and that they decided it was close enough to save them the cost of commissioning art specifically for the book. Of course, since The Man in the Iron Mask is a classic it doesn't really need a flashy cover to sell it. Indeed, I bought this book despite the cover, not because of it; I was at the book store specifically looking for The Man in the Iron Mask. Most books don't have this level of fame going for them, and I have to wonder how many more people might have bought this book if it had a better cover design to catch their eye.
On the other side of coin there are books whose covers are so appealing that I'm predisposed to buying them before I've even read the blurb to find out what they're about. Mind you, a well-designed cover should give you a good idea of what the book is about before you even pick it up.
Here's a book I bought last week solely because of its cover:
It was the layout of the book more than the actual illustration that grabbed my attention. This book makes an excellent argument for self-publishers to hire a professional cover designer instead of trying to make the cover themselves. The cover alone translated into a sale.
Here's another book I bought about a year and half ago based solely on its cover:
I never get tired of looking at the illustration of this book; not only is it very clever, it communicates a great deal of information about the book. This is probably one of the best cover illustrations I've ever seen in terms of its aesthetic appeal and implied narrative.
I'm very pleased to say that both of these books lived up to the promise made by their covers: excellent stories that were every bit as much fun to read as their covers suggested. So sometimes I guess you really can judge a book by its cover.
I hadn't really thought very deeply about what makes an effective cover when I was forced to design one for Libram Mysterium; my ruminations have been subsequent to the book's publication. When I did the cover I was rushed and behind schedule and trying to juggle too many irons at once. Nonetheless I'm quite pleased with how it turned out and somehow, by intuition, I managed to produce a cover that is visually striking and is faithfully suggestive of the types of stories that the reader will find within. Of course I was very fortunate to have Christopher Conklin as the artist. He not only drew all of the interior illustrations, but also made the cover painting on very short notice. One of the truly groovy things about working with Chris was his ability to take just a few lines of description and produce a picture that was exactly what I wanted. I'd tried to keep my instructions minimal and somewhat vague in order to give him creative space, but he nailed the image I had in my mind's eye every single time.
In any event I feel more confident that I understand the elements that make for a great book cover for the next time I design one. Acquiring the technical skill to pull it off is another matter...