http://covers.fwis.com is a wonderful blog site focused on contemporary book cover designs. Or, in their own words, "Covers is dedicated to the appreciation of book cover design."
I frequently give my 2 cents on the site, but mostly the site is very diverse in it's representation of good book covers—thereby attracting a diverse quality of discussion. There are a good number of features and sometimes the actual covers designer will comment and explain their solutions...
Other good places to dive into some good talks about book design or just design in general are:
http://foreword.ospreydesign.com/
http://nytimesbooks.blogspot.com/2006/12/my-favorites-of-2006.html
Enjoy...