The My Penguin series of blank-covered classics, for which you can design your own cover, are a fun idea (see here for details). Penguin themselves decided to promote the series by having a bunch of bands/musicians design covers for some of these seminal books. Sadly, they picked a bunch of (mostly) third-stringers, and some of them are even less inspired at visual design than they are at music.
Look at these, for example. The first is Razorlight's cover for The Great Gatsby, while the second is Ryan Adams' cover for Dracula.
Razorlight's singer says: "I was running a book on the grand national in Tokyo and I was writing out a betting slip for everyone, and decided that my bookie's name was going to be Gatsby. The cover of this book is a betting slip." Oh, good. So you didn't just throw together a piece of crap in 30 seconds, then. And Adams has produced a something that looks as though it was made by a 3-year-old out of poo.
As an example of how this might be done right, see Beck's contribution.
For another good example of My Penguin cover art, see the great Eddie Campbell's take on Aurelius' Meditations and Austen's Emma.