Two different exemples:
Another book cover. I was on the best place in Champaign-Urbana, Barnes and Noble, and I thought this cover really cool. The function is obviously, captivate the costumer and sell the book. I do not know this book and the author, but the concept created to the cover tells me that the content should describe a person without limit. Someone who has a infinite brain, as the universe. Someone who has several stories to tell. Then I looked the review behind, and I read exactly what I thought when just looking the cover. Maybe the function worked, because I am interested in the book now, the cover led me to the review and now I see the real function of a interesting and quality cover.
The second cover has the same function, sell it. But the argument is different. We see a new book, printed on the last weeks but with a rustic and 'out of time' cover. Why? It is part of a classic collection of books, they are pretty when they are together. The cover recall a old and classic time. This kind of cover makes you feel as you were back in the past. For those who love the classics, like me, it is pretty easy to be convinced to buy them.