Enabling a revolution in digital reading, Sony today announced that as of May 5, book lovers and eBookworms have downloaded more than 10 million books from the Reader™ Store. This does not include the millions of books downloaded from other sources such Google Books and public libraries, or through other ebookstores in the U.S. or Sony affiliated bookstores in Europe. In celebration, all accounts with an eBook purchase from the Reader Store on May 5, 2010 will receive a $10 credit for future purchases on the store. The 10 millionth book downloaded was “Digital Fortress,” by Dan Brown.
Note that they're not boasting of 10m _sales_, but downloads. Sales are probably only a quarter of that number.