I've seen several writers quote Bowker's figures on ebook sales, which say they only account for the smallest fraction of books sold and money made. The problem with this is that Bowker can't track Amazon Kindle sales. Only Amazon can, and Amazon is not giving that information to Bowker. While Bowker can track sales based on ISBNs, Amazon ignores ISBNs in favor of it's own Amazon Standard Identification Number.
See this blog post by Steve Weber, author of Plug Your Book, for more information: http://www.weberbooks.com/2009/05/will-kindle-crash-nielsen-bookscan.html