Seattle-based Amazon.com has a very interesting political heat map of the country, showing the proportions of “red” books and “blue” books purchased by its customers in each state.
It’s updated daily. As of this writing (8-22-12), so-called red (Republican) books are outselling blue (Democrat) books nationally 56%-44%. My home state of Washington is slightly more even, but red outsells blue 54%-46%. The extremes: Washington D.C. buys 76% blue; Mississippi buys 73% red.
More details (including which books Amazon considers red and blue): http://www.amazon.com/gp/