TitleZ makes it easy to see how a book or group of books has performed over time, relative to other books on the market. Simply enter a search phrase, book title, or author, and TitleZ returns a comprehensive listing of books from Amazon along with our historical sales rank data.
I signed up for an account with their beta release and performed a keyword search on Texas. It returned several hundred books ranked by sales rank. When I clicked on the left hand button, it showed a neat historical chart of the book's sales. Pretty neat. If you visit the site, don't forget to check out the compare feature. The beta release is free, but they plan to charge a monthly subscription after its goes prod.