Until a few years ago - so before the Kindle era - I just LOVED to browse around book shops and inevitably came out with a couple of books in my bag!
So what made me buy those books? Well, to be honest, first I looked at the cover. Admit it, a beautiful and attractive cover makes you want to pick up the book! Only then you read the cover text and perhaps have a peak into the first pages (and the last ones, in my case)
That is why romance novels all have those gorgeous women (and men) on their covers. The heroines in those books are mostly slender females (well, that never gave me cause to start a diet) and their beaus are alpha men.
That is why a cover is very important for a book. Speaking from my own experiences, I am asked by my publisher what I'd like to have on the cover. It should also tell something about the novel. So in the case of The Medici Diamonds I needed to have the diamond necklace on the cover, because this necklace plays an important role in the development of the story. And you'll also see a glass of spoilt wine, which points to the beginning of the novel. With Maria Gonzalez I asked the cover artist to pay close attention to Maria's blue eyes, because they also are important and the reason why the Aztecs save her life. See what I mean?
So readers, what makes you buy a book? The cover? The contents? The author? Don't hesitate to let me know in a comment.