The last (january) french edition of advancedphotoshop, a magazine for design professionals and passionates, has a very interesting article on the design of book covers : the prerequisites, the rules around it, some pieces of advices from professionals and a couple of exemple.
This reminded me of a great collection from penguin books : My Penguin.
The idea behind that is to sell at reasonable price classics with a blank cover, to let the customers design it by their own taste and then to publish these covers on the My Penguin website..
It also highlights covers from famous artists.
Although you can no longer submit your covers, this kind of offer shows the innovative approach of Penguin Books to get in touch with the readers