If you notice to the right, I've installed a new widget from Amazon. It's a book carousel displaying my four novels and six anthologies in which I have stories. Go ahead and spin it. It's pretty cool.
While playing with the carousel, I noticed something quite striking about the book covers. Some of them are easy to read, even as thumbnails. Some are not. This differences provide a good lesson in effective book cover design.
No matter how evocative the art when see full-sized, no matter how clever or interesting the font, if it can't be seen as a thumbnail, it isn't a good design.
Black Dragon, White Dragon has a cool cover, but as a thumbnail it is too dark and you can't read the title. Tales of Fantasy has a better title design but suffers equally in the art department. The Thieves' Guild has evocative art, but looking at this thumbnail you would think the title is Crossroads, if you could read it. Still, it's not a bad cover. The Rose and the Skull, Relics and Omens, and Rebels and Tyrants all fail artistically because they either fail to evoke movement (R&T) or are too small and detailed to get across the message. Relics also has poor font design. Warrior Wisewoman has a nice, clear, evocative image, but the title gets somewhat lost. The Futures from Nature cover looks like it has a pile of shoes on the cover.
The best covers are Dark Thane and Conundrum. The titles are easy to read, the images easy to see. Dark Thane has a dark cover, but the artist has nicely contrasted the darker elements with a light background. There's a lot of action and conflict suggested in this cover. Conundrum is also good, even though the interesting parts are small and still, because they draw attention to themselves through the separation and strangeness of asymmetry, almost like a zen watercolor.
But a thumbnail is no way to judge a book by its cover, says you. What better way to judge it, says I? The purpose of the cover is to attract and hold the attention of a prospective buyer. You only have a few miliseconds to grab their attention as they walk through the bookstore or browse the website. Wherever they see the book the first time, more than likely they will see it about the size of a thumbnail. On a website it will definitely be a thumbnail, but in a bookstore it will appear to be about the same size, when seen from 8-10 feet away. So a book's cover simply must work in thumbnail size, if it is to grab the reader's attention.
To accomplish this, you need a clear, easy-to-read, yet unique font, and iconic artwork that is simple and engaging, easily identifiable, and instantly evocative.