An interesting website that is currently in Beta testing mode is Titlez.
The site offers a way of tracking the Amazon.com sales ranking of one or more titles. Though Amazon updates their sales numbers hourly, and the rankings are a useful way of tracking a book's popularity with the world's largest online retailer, they keep no record of the title's past success.
Titlez.com collects and graphs Amazon rankings over time. Just type a title, subject, publisher, or author name into the search field, and you'll see a graph that features the day-by-day performance of a given book.
You can find things like a book's best rank, worst rank, 7-day, 30-day, and 90-day ranking, and lifetime average. You can also compare the performance of two or more titles.
Since the site is still in development, not all titles have information collected, but you can request a title be added. (Clinton's book, for instance, had the information readily available. I had to request that they track the sales figures for Geoff's book.) For now, access to the site's resources is free -- you just need to register with your name, e-mail, and a password. Pretty cool, huh?