This from today's USA Today: (bolding is mine)
'Kindle sales: Starting today, USA TODAY's Best-Selling Books list becomes the first major list to include Amazon Kindle e-book sales. The move reflects both the growth of e-book sales and Kindle's role in that market. "Since 1993, USA TODAY's Best-Selling Books list has always evolved to reflect the ways our readers buy books," says Susan Weiss, managing editor of the Life section. "Adding Kindle to our group of contributors makes sense given the growth in the e-book platform." E-books, for all devices, claimed 4.9% of sales in May, according to book audience research firm Codex-Group. That's up from 3.7% in March. This week, Barnes & Noble announced the launch of its own eBookstore with 700,000 titles."