How many times have we heard that cliche? Don't judge a book by its cover. Don't judge someone by how they look on the outside. Don't judge a situation when you don't have all the information.
This is great advice for life situations, but when it comes to choosing books, we, as readers, are tremendously shallow, because we have to be. We browse books selectively, automatically putting aside anything that doesn't fit our visual requirements. Unless you show up at the bookstore or log onto Amazon knowing exactly which book you're looking for, chances are you are skimming book covers and titles, waiting for something to grab you.
Book covers tell us what we can expect to find inside. Will it be scary, sweet, contemporary, historical, romantic, exciting, scholarly, futuristic, paranormal, epic, contemplative, funny? We set our expectations based on the colors, images, fonts, and composition of the cover.
A pastel background with a curvy text surrounded by daisies? Might be romance, might be women's fiction, might be historical. Definitely won't be sci-fi or horror. If you pick up this book, begin to read, and find zombies chasing you through a deserted warehouse on page three, you are going to feel cheated. Not that there's anything wrong with zombie books, but you didn't sign up for the living dead. You were probably hoping for a love interest with all his own body parts.
What happens next, after you slam the book down in disgust? You leave a bad review. Not what I expected, you'll say. Totally misleading cover, you'll type furiously. This isn't good for either the writer or the reader. We all want a good experience, on both ends of the book cycle.
Writers, choose your covers carefully. If you're self-published, consider getting some help with your cover unless you have some sharp graphic design skills. If you're traditionally published, pray that the powers that be share your vision for the book. And readers, always read the blurb.