Who says that Book Cover must be essentially a static picture? In this Electronic New Millennium, do Books mean only P-books? If you take it for granted that Print/Paper/Physical book is the only 'real' Book, then stop reading this post for a while e.g. until the next sunrise.
In my imagination, the E-book has the potential to raise (and revolutionize also) the status of the Book Cover concept. If comparing to the last century books, we now see a sea change in the books' appearances in terms of the arrival of just a Graphic on its cover, why not visualizing even ahead especially in this electronic era? An era where we can insert Slideshow, Moving GIF, Audio, Video, etc. in place of the cover art!
What was the purpose of the static Graphic on the book cover, in the very first place? It was to attract the Audience and to make them remember the Book in the long run, even if they didn't read it, and even if they forgot the title! Just as we might forget the names of people we are not in contact with and yet remember them by their faces, even over a long time. And the people with beautiful faces have their names hitting the public memory in the higher likelihood.
Again in the Internet, currently GIFs and Videos are overriding the static photos in many places. So to create more appeal, to show more beauty, we may shift to a sculptural effect by inserting Moving GIF or Video clip! And making videos are not costly technically these days.
A compact video clip of say 2 to 3 minutes may do a wonderful job of N numbers of beautiful cover designs/photos presented in a combination. To add further in this synthesis of visuals, contributes the sound of the Video. The Author may himself/herself speak here, with their own voice modulations, to create extra beauty or extra appeal.
GIFs are also good as they repeat certain Movie-clip Themes, like the Pop music where the repetition does give rise to a Beauty.
Audios can also be added into a Static graphic, or even better, into a Slideshow.
Thus, if you consider P-book as the only real Book then you are missing out these fascinating opportunities. To endow the concept of Book with a NEW dimension. From the angle of the Cover Art, as they say. Where this ART manifests (fully) in the various forms of the Art itself.
This New Dimension is possible only in case of the E-books! So to me, it seems that E-book is the 'only' real Book! And from now on, I would like to equate the word Book to E-book (by default).
And the paper version? I'd never call it Book exclusively. I'd rather say: P-book. As though a new term P-book is born if we look back in history into the last Millennium!♦