The timeworn adage asserts, "You can't judge a book by its cover". True enough. But if it's an effective cover in its design and content, it will provide a great deal of information about the book that will help the viewer in deciding whether to read or purchase it. For self-publishing authors, and for any authors who are able to exercise some control over the design of their book covers (because they either design the covers themselves, or hire someone else to do it with whom they're able to consult), it's essential to know what makes a cover effective. While I am going to focus primarily on books printed on paper, many of the principles discussed will apply to designing and creating covers for e-books as well.
Covers are extremely important: they can visually establish a "branding" icon for a book or for a series; they become identified with the text in readers' minds (recalling one will recall the other); and they help to sell copies. In the marketplace, I think poorly designed covers far outnumber those that are effective. This is a pity. But it does give those books with well-designed, effective covers a marketing advantage. An author may get only one shot at marketing a particular title, and a good cover may go far toward contributing to the success of that effort.
In designing a cover yourself, or in hiring a professional designer to do the job, there are several key principles to keep in mind which will serve as guidelines for planning and executing the cover's composition, and for evaluating its effectiveness.
First, it's an error to conceive a cover's overall design primarily as a graphic composition (a "pretty" picture) rather than as an esthetically-pleasing tool with work to do. This is a mistake that many designers make.
For, to be effective, a book cover must accomplish a number of objectives: it must (1) catch the potential reader/buyer's eye, (2) pique that person's interest and curiosity, (3) represent fairly the nature of the book (i.e., be congruent and consistent with the book's subject matter, theme, content, etc.), and (4) provide basic information useful to purchasers, reviewers, booksellers, and librarians (title, author, price, bar code, ISBN, publisher's imprint, blurbs (if any), and category ('a novel', 'poetry', 'how-to', 'humor', 'scholarship', biography, memoir, etc.)--all of this in a clear, uncluttered, legible, and quickly-graspable fashion. That's a lot of work to do.
In short, a cover is specifically designed to call attention to the book, attractively present it to the public, and convince the viewer to read or buy it. Visually, it must be sufficiently striking to stand out among (perhaps) hundreds of competing covers displayed on shelves, racks, or tables, causing the viewer to focus on it (even across a crowded room) and say, 'Hey, I want to check that out,' and go pick it up. A badly-designed cover can actually hinder sales.
Like record jackets and posters, book covers are a specialized and complex art form. They combine esthetics and "grab" with workaday information. [I think of Alphonse Mucha's work, Toulouse-Lautrec's and Maxfield Parrish's advertising art, and James M. Flagg's Uncle Sam pointing at the viewer to say "I Want You".] Esthetically, a book cover must satisfy three audiences: (1) the artist who designed it, (2) the author whose book it's announcing, and (3) the publisher, whose taste, savvy, and professional competence are being represented in the marketplace and to the world at large. If the author is a self-publisher, these three audiences may be one and the same.