"Don't judge a book by its cover" might be a frequently referenced proverb, but it's completely false. Everyone judges a book by the cover and there is an entire industry devoted to making sure the inside matches the outside. It's a visual code which is so ingrained that most people probably never realize how much it influences their choices.
Take a look at these covers:
Without needing to know anything else, I bet you could pick out the non-fiction book with it's mostly text cover. The children's book is another easy gimme, with its paler colors and an illustrated cover. Maybe you hesitated at the third one, wondering if it was science fiction or a young adult. Surprise, it's both. And the attractive young man on the fourth is promising a happily ever after before the final page.
Choosing the elements of a book cover goes far beyond simply making sure it matches the contents (like not putting a racing car on a medieval historical). The colours have to match the tone of the novel (urban or rural, dark or optimistic). Taking a look at the two romance covers below, it's easier to see which has the darker and more sensual tone. A reader would not pick up the left hand novel expecting a twisted and tortured hero or heroine.
I'm not a slouch or newbie when it comes to Photoshop manipulation, but I would not try to design my own cover. Instead I hired a professional from Streetlight Graphics, someone who understood the inherent language of covers.
They delivered a cover which captured the essence of my novel and provides important visual clues to potential readers. My heroine's level of dress shows that the book falls in the moderate range for sensual content. The dark colours and cityscape elements identify it as dark urban fantasy. The snake provides an interesting teaser, while still hinting at a dark overtone. Even the blue wash provides an important cue, suggesting a hint of science fiction rather than fantastical magic.
The cover is an author's statement and lure. "I liked the cover" is still the reason most readers give for picking up a new author or novel. Once inside the pages, the cover has set the reader's expectations, and they will be bitterly disappointed if those expectations are not met. Most online shoppers aren't looking at descriptions, only scrolling down through pages of thumbnail covers, waiting for one to catch their attention.
After months and sometimes years slaving over making sure each word is right, it can seem unfair to need to dress up a masterwork in colours and flashy text. And it is unfair that a cover can make or break an author's chances of finding their readership. Most of the time, the author is only minimally involved in the cover, even if he or she has final approval of the design. So why does the cover rule?
Because humans are visual creatures and respond faster to a visual than to a story. Stories take time to take up residence in the human mind, whereas images are branded in our memories in fractions of a second. We can register an image faster than our brain can recognize we've seen it (which is why sponsorship of racing cars works). Humans still shop like we used to hunt and gather, scanning the surrounding area with only half our attention until something jumps out. That's why it's called "catching" someone's attention.
Like it or not, the cover is an author's hook, reaching out to snag readers with colourful fingers. Without it, we would likely pick up entirely different selections. There is a novel at the library which I pick up from the shelf almost every time I go, despite the fact that I've read it and it was so bad that I didn't get past the first few pages. Yet every single time, something about the cover snags my attention and I have a few moments of intrigue before I recognize it and put it back.
It just goes to show, a great cover won't save a horrible book. But a great book with a horrible cover will never be discovered in the first place.