|Maybe it has a spine.|
According to Ms. Chittenden, designers and art directors should put as much time into designing a book's spine as they do the cover. In bookstores most books are on shelves. They're displayed with the spine out ... not the beautifully designed cover. When the book browser, and hopefully book buyer, is walking along the shelves, the spine needs to grab their attention. The book's spine design will get the book browser to pull the book from the shelf and see that well-designed book cover.
Panzer says, "Makes sense for dead tree books ... but do my ebooks have spines?"
To read the post, tootle over to book spine