Bonus update (original posting below).
In addition to the quote by Sean Penn in Entertainment Weekly cited below, Sean also talked about the book cover for Jon Krakauer's Into the Wild in a recent interview in Time magazine:
"The cover grabbed me--the bus, the image of the bus with the title Into the Wild on it. I've made a lot of decisions in my life that you could call judging a book by its cover. And I've become a real advocate of it. So I took the book home, and I read it cover to cover twice, and I went to sleep in the wee hours and immediately got up in the morning, and I saw in essence the movie that you saw last night."
I noticed the following quote from actor and director Sean Pean in a recent issue of Entertainment Weekly. He had been asked when he decided to make Jon Krakauer's Into the Wild into a film. His answer was as follows (the bold is my emphasis):
"When the book first came out, I wandered into a bookstore, saw it on the shelf, judged the book by its cover, took it home, read it twice, finally fell asleep, woke up, and started trying to see if the rights were available. I had a very strong feeling that this thing was dying to get out of the pages and onto the screen."
That's how a cover should work. It should draw potential readers into the book, get them to pick it up, buy it, and read it. That's what happened to Sean Penn. That's why covers are important, not just for the initial sell, not just for the initial impression, but also for the potential of follow-up sales, including movie rights and other subsidiary rights.