Authors, now when you log on to Amazon Author Central they give you sales stats for your books. They even have a neat report that breaks sales down by geographical location. It looks like my aunt and uncle in Michigan have been buying lots of Alfie books lately. :)
The data is coming from Nielson Bookscan so it's not just Amazon sales. They claim it represents approximately 75% of all print book sales (both online and instores). Pretty neat stuff.