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

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.