Seth Godin (internet marketing guru and all around interesting guy) weighs in on book covers in a recent blog post.
Tactically, the cover sells the back cover, the back cover sells the flap and by then you’ve sold the book.
This is a new thought for me, and an interesting one. I’ve found that one of the easiest ways to tell a good designer from a great designer is by the quality of their back covers. Mine are fairly horrible. Something to work on.