An interesting question posed by Mistina Picciano of Market It Write:

http://marketitwrite.com/blog/2011/09/are-naked-books-the-new-design-trend/

Creating cover art for your ebooks does three things psychologically:

1. Makes the invisible ebook visible, so that people become aware it’s available, and what it is – in an instant.
2. Packages the book to make it seem more real and to add perceived value to what’s essentially a computer file.
3. Identifies the book as a part of your brand and hooks it in more firmly to your brand’s overall message. This makes your ebook more of a “team player” in your brand.