In 2004, 950,000 books out of 1.2 million tracked by Nielsen Bookscan sold fewer than ninety-nine copies. Only 25,000 sold more than 5,000 copies. The average book in North America sells about 500 copies
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Miss Snark discussed these numbers on her blog. The short version is that the numbers are misleading because Bookscan tallies ISBN numbers, and a book may have many ISBN numbers. Bookscan also doesn't take into account library purchases or Wal-Mart purchases. Nor do the stats cover paperbacks, which is a pretty big chunk of the market. I would have been interested to know if it included Book Club sales or not, but I wouldn't be surprised if it didn't.