I think it may be easy to infer that the main job of a book cover is to encourage the reader to pick the book up. But, if the design, artwork and information developed for the book's sole goal is to encourage the reader to pick the book up and, ostensibly buy the book, this might infer that it's okay, maybe even encouraged to "sell at all costs". I write to highlight that point as it may the sole distinction between the other answers and mine. I certainly do not disagree that the design of the book cover plays a role, often a critical one in the sales of the book. But more importantly, the design and the communications goal are not always monolithic when approaching a book cover design. Things like audience, author preferences and other factors play a major role in book cover design. These things need to be researched thoroughly and are ignored at the project's peril. An example of my point is the notion that an unknown book about politics and a book from a known political figure have different communication strategies.
The "known" person can take advantage of name recognition and in that case the design or action of the book has to treat that name recognition dynamic as a design element. Conversely, the "unknown" writer may need to focus on the subject matter, creating some type of interplay that allows the subject matter to be understood quickly and poignantly. Looking at movie posters over the years, the same type of dynamic exists. So, I think, no aspect can be assumed. Book sales? What if the book is offered online primarily or self published? What if the author is well-known or a best seller? The design could be perceived as being "in the way." This is why designers exist… not simply to sell products. But to uncover and answer questions and solve problems. As Don Draper, the creative director character from Mad Men says: "I sell products, not advertising." Don't lose that focus.
Lastly, the role that book covers play in selling a book can vary completely based on the above information. Graphic design is for this reason a study in understanding the client's needs and wants as well as a translation of the reader (or purchaser's) needs and wants. Mirko Ilic author of Design of Dissent, said that designers have two clients: the client who pays and the client who consumes—sometimes not the same people. It's important to remember that there is a fine line that must be drawn between a work that accurately distills the client's needs, requirements and wants and develops them into a work that can reflect the reader's needs and wants. A foot in either bucket too much reflects work that can result in flawed work for reasons we've all seen.