Nielsen BookScan USA

BookScan – the world’s first and largest continuous sales monitoring service

BookScan provides weekly point-of-sale data with the highest possible degree of accuracy and integrity. Functioning as a central clearinghouse for the US book industry, BookScan enables its subscribers to access comprehensive reports from a wide variety of perspectives.

BookScan US provides a continuous market measurement of US retail book sales based upon electronic sales data analysis. Basically, we collect point-of-sale information from a variety of retailers and make this information available to the industry. In a typical week, sales of over 300,000 different titles are collected, coded and analysed, producing complete market information for retailers, publishers and the media.

Also: http://en.wikipedia.org/wiki/Nielsen_BookScan and http://www.slate.com/id/2142810/