From The Ugly Truth About Getting Your Book Published

3. Average book sales are shockingly small, and falling fast.
Combine the explosion of new books with the declining total sales and you get shrinking sales of each new title. “Here’s the reality of the book industry: in 2004, 950,000 titles out of the 1.2 million tracked by Nielsen Bookscan sold fewer than 99 copies. Another 200,000 sold fewer than 1,000 copies. Only 25,000 sold more than 5,000 copies. The average book in America sells about 500 copies” (Publishers Weekly, July 17, 2006). And average sales have since fallen much more. According to BookScan, which tracks most bookstore, online, and other retail sales of books, only 299 million books were sold in 2008 in the U.S. in all adult nonfiction categories combined. The average U.S. book is now selling less than 250 copies per year and less than 3,000 copies over its lifetime.

4. A book has less than a 1% chance of being stocked in an average bookstore.
For every available bookstore shelf space, there are 100 to 1,000 or more titles competing for that shelf space. For example, the number of business titles stocked ranges from less than 100 (smaller bookstores) to approximately 1,500 (superstores). Yet there are 250,000-plus business books in print that are fighting for that limited shelf space.

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