The effect of the use upon the potential market for, or value of, the copyrighted work
I would think the effect of copying the cover of a book on a post discussing the movie on the potential market for the cover art is exactly zero. The image has already been licensed for the book cover. Copies of the book cover appear in ads for the book in scads of places. (Amazon. Links to amazon. other book stores etc.) It is possible that in future someone might like the image. But since it already appears as the cover of Freakonomics, that person would either (a) decide to not use it because it appeared on the cover of Freakonomics or (b) use it because it appeared on the cover of Freakonomics. In both cases, the fact the the cover of the book appeared in a blog post or similar would be discussing Freakonomics itselfirrelevant to whether someone would want to license the image for some other use.
So... zip.
The analysis might be different if someone was selling t-shirts with the image or selling a second book with the image and so on. But really... there must be some sort of implied agreement that people can show pictures of the book when discussing the book or movies about the book!