As someone pointed out in the comments, these figures ignore self-published ebooks that don't have ISBNs and thus don't show up in Nielsen, BookScan, etc.<p>Most U.S. self-published ebook authors don't use ISBNs because they're outrageously priced in small quantities, and none of the major ebook outlets (Amazon, B&N, Apple) require them. The situation is somewhat different in other countries, where ISBNs are issued by government agencies for free.<p>These books are a <i>substantial</i> portion of Amazon's ebook sales, the majority in some genres.<p>For a more comprehensive take on sales numbers, see:<p><a href="http://authorearnings.com/" rel="nofollow">http://authorearnings.com/</a>