It is a good practice to think about this, when you are trying to decide how you want your book cover to look. I tell authors to go to their local book store and walk up and down the isle of books similar to their book. What attracts you? What makes you pick up that certain book and flip it over to read the back cover? Do not copy someone's cover, but use the energy that you feel to fuel the fire for your own cover.
Another great source is the magazine rack at your local store. Those companies pay someone to create a cover that makes people want to purchase that magazine. Take a careful look at their "Look." Ask yourself: Why did they use that color, font, style, or way of presenting the material?
The same is true on the internet, probably more so. That's because everything on the Net is two dimensional. If you want to make a good first impression, to draw someone in, then you have to present your product in a way your customer can relate to.
You have to present it in a way that makes a person want to reach into their screen and pick it up. If you can do that, you have done a good thing. Then you will make more sales. It's as simple as that.