In the world of objects, the cover plays an important role. For books, the cover is more than just something to wrap and protect the content. A cover is a sales pitch, a tease, a distillation of the story. Great covers are beautiful and thought provoking. The best might be considered art. Once a book is purchased, the cover is a gateway, something that must be passed through each time you enter or exit the text. When you're finished reading and you place the book on your bookshelf, the cover becomes part of the book's souvenir value. You display its spine. You might take it off the shelf and admire it every now and then, even if you don't read it. So it's interesting that digital books place so little emphasis on the cover. Sure Amazon has the cover there in icon form as do most ereader apps for tablets. The Kindle and other eink readers barely bother with the covers at all. To even look at one requires backtracking through the front matter from page one (where the Kindle assumes you want to start reading).
Book designer and writer Craig Mod recently ruminated on these matters in an essay posted to his web site. He concluded that we should stop drawing direct comparisons between what a cover does in the digital and analogue worlds. The digital cover is a different beast entirely and we need to think about how to best design for it. "Hack" is the terms he uses, actually. A simple point of difference that hasn't been exploited yet is the ease with which covers can be updated and that readers are often alerted by their devices when such an update occurs. When a new book is released, why not update the cover for all the backlist, letting your readers know you have a new title available?
Another interesting approach is to exploit the way books are displayed in ereader apps. Making a series of book covers fit together like a jigsaw doesn't exactly work on a bookshelf, but it looks amazing in an iBooks or Kindle app library where all the books are displayed face out.
If, on the other hand, you just want to revel in great book design and listen to someone who knows a thing or two about it, go no further than Chip Kidd's highly entertaining TED talk at http://bit.ly/ifbook148