I ran across this interesting example of what went into a book cover, namely Michael Crichton's Next. Part of it says, "on every big cover you have a million opinions. You ahve the editor and the editor-in-chief and the publisher, and you show it to the author and he's OK with it. ... The jacket has to have the blessing of key store accounts, espcially that of Sesalee Hensley" the fiction buyer for Barnes and& Noble.
[Full disclosure: It's kind of weird and scary to think that one person is having that much impact on what America sees. I was also reading about how the Advanced Reading Copies of a well-known author were released with one title, which was changed when a buyer (possibly the same person) didn't like it.]