But of course, we all do it. Think about the last time you strolled through the aisles of a bookstore (if you still have access to one) or in the library, glancing at the hundreds of thousands of titles available to you. Something on the spine of one causes you to pause, to slide your fingers onto it, pull it out for a better look... hmmm, the image(s) on the cover look "wrong" for your mood... you push it back into the stacks and continue your quest... aha! Here's one... you free it from its confinement, letting the cover see the light of day, perhaps for the first time... hey, that looks cool... into the basket it goes. And so on.
Once upon a time, book covers were limited to a title printed in block caps, not even an author's name. Maybe some gilt if you were really lucky. Then things got a little fancier, and certain top-shelf authors such as Mark Twain and RL Stevenson got "special editions" with nifty one-color graphics PLUS the gilt. Woo, hoo.
Then came the penny dreadfuls, and the pulp fiction of the 20th century, with their full-color but bad art. When speculative fiction became a "thing" in the mid-20th-century (of course, it had always been around under other names), a new art form was born: sci fi/fantasy art created especially for the novel within the pages. Some of it was weird, some of it was beautiful, and nearly all of it was eye-catching. These covers began making other fiction look plain by comparison.
Not to be outdone, the romance biz, long a bastion of luridness, came out with photo-realistic covers featuring gorgeous male chests and scantily clad heroines that more than hinted of the eye candy within. Fast forward a couple decades, and in the era of the ebook covers are now a centerpiece of the marketing for a new title. We have "cover reveals" for forthcoming novels months in advance of their release. Jpegs get passed and posted all over the Internet. Covers are king.
Of course, the hype is exactly that. A great cover can help stimulate interest and hopefully boost sales, but if what's inside is terrible it's still a bad book. What's more, readers have not the slightest clue or interest in what is involved in designing that fabu-licious cover. They want a good book. To readers, covers are branding, and branding is the promise of a product to live up to the consumer's expectations. The cover is the unspoken message explaining genre, protagonist, setting, and content details (is there action? romance? paranormal elements?). The book had better deliver on that message.
I've been thinking about covers because I'm in the process of finalizing my own with my editor. It's been an interesting and fun process. I've looked more closely at covers than I've ever done, and it turns out that while there are several formulas, there's no hard and fast rules, even within a genre. I can say that with the advent of low-cost, high quality stock illustrations and someone clever with design software, producing a great cover is cheaper and easier than ever. I doubt that specially-commissioned cover art is going away for top-selling authors, but I imagine that new authors, mid-list authors, and small presses are going to be producing their covers entirely digitally. In a way, we're back to the late 19th century.