The most reliable way to find out how many copies of a book have been sold is the Nielson BookScan service. It is used by the publishing industry and media to track book sales on an ongoing basis.
BookScan gets its data by tabulating total sales from retail book sellers. As of 2013, with the addition of statistics from Walmart retail outlets, BookScan claims that its data includes about 80 percent of retail sales in the United States. The data are first divided into primary headings such as adult fiction, adult nonfiction, children and other. They are then subdivided into more precise genre groups. Because the service is expensive, it is not used by many individual authors. However, starting in late 2010, Amazon made BookScan statistics available as a free service to authors through its Author Central portal.
Although BookScan is used extensively throughout the book publishing and distribution industry, it has been criticized for its inaccuracy. Publishers are the only truly reliable sources of accurate book sale statistics, and they do not release sales information to the general public. Additionally, BookScan does not report on digital e-book sales, which account for a growing percentage of the book-selling marketplace. However, in the absence of alternatives, despite the limitations of the service, as of 2013 the publishing industry and media continue to use BookScan.