The top three liberal books are all by Senator Obama. The top three conservative books are more of a grab bag including one about Governor Palin, one by former Clinton aide Dick Morris, and one authored by Chuck Norris. John McCain's book ranks sixth but hardcover and softcover are counted separately.
The "Purple" book category features less than partisan books. Tom Friedman's book on the United States, globalization, and the environment is number one in the purple category. I have not read it but I am told geographers would find it interesting.