I feel as though book covers are more likely to depict and represent what the novel is about whereas movie posters are more for marketing something and therefore adhere to certain standards. Many movie posters for the same genre will look the same, and while book covers can too, I feel like with movie posters you see it much much more.
This article actually talks about how a book cover for the book "It's a Man's World" ripped off the movie poster for "Morning Glory".
Through my Google search I also came across this article at Smashing Magazine regarding the best movie poster designs that I thought was cool.