Moira Allen wrote a wonderful book recently that included information on how to impress publishers, editors, and agents with your understanding of the book market. She said something I have seen in other writing blogs since then: sometimes it pays just to check out the local Barnes and Noble and see what's on the shelf.
This advice is true no matter what stage of market research you are in for your book – topic, competitor authors, size of audience, and so forth. It’s especially true for book design research, which includes researching your book’s market regarding book covers, layout, what’s included on the inside flaps, and so forth. It’s important to know what others in your space are doing with their covers, what works to attract a reader’s attention, and even what’s expected or typical in terms of layout and what to include on the inside flaps.
People care about how a cover looks. They look for sharp, clean designs, uncluttered imagery. They look for images that visually describe your book. They also look at the information on your cover (including inside flaps and back cover). Notice in which circumstances the title or the author name is the focus. Check out when the image or text has the priority. See if the cover is in color or black and white. If in color, how many colors are used?
Finally, is the cover copy just a series of quotes praising the book? If so, where do those quotes come from? Are we talking New York Times book reviews, or the author's neighbor who works at the newspaper in town? If the cover copy pitches the story, does it overwhelm you with details and names, or does it focus in on one main question?
So look around and see what others in your market are doing in these areas. See what’s selling and what’s not. You don’t want to duplicate what’s out there, you should be unique. But these considerations should factor into your decisions about your own book design. Don't research your competition just to say, "I could do that." Think about what actually works in your genre, and find ways to make yourself more marketable with every trip to the bookstore.