Richard Minsky, of American Decorated Publishers' Bindings, 1872-1929 fame has started a new blog, American Book Covers.
Here are his comments on what he plans to do:
I started a blog on book cover art, and in the sidebar have feeds from six other blogs that primarily feature book covers. If anyone thinks that no art has been done on paperbacks or dust jackets in the last 20 years, check out the selections of those bloggers. For my part, I am not adding competition to their well-covered territory, but will present selections from the thousand or so book covers that have been in my exhibitions. To start with I show an anonymous cover that is done in printed paper wrapped boards from Houghton in 1881 with a design that anticipates constructivist, futurist, and abstract expressionist paradigms.
Take a look and be sure and bookmark what is likely to become a useful reference.