Gotta differ on the book by its cover routine. Typically, if a publishing company thinks the book is good, they'll invest in a good artist to do the cover.
Hehe, now, it shouldn't be the entire basis of one's decision, but always consider the cover when buying the book of an author whose works you aren't really familiar with.
There are quite a few books I've bought on the basis of the description on the back, the cover art, the publishing company (Tor, for most of them) and the thickness of the book. And I've got to say that most of those rank pretty high among my favorites, especially C.S. Freidman's books.
Anyway, never judge a book SOLELY on its cover
