Amazon, which does not divulge exact sales figures for the Kindle or e-books, said that for every 100 print books it has sold since April 1, it has sold 105 e-books. That includes both paperback and hardcover books, but excludes free downloads.

Last month, Amazon introduced a Kindle for $114, or $25 less than its next most expensive version, featuring advertising.

The Kindle has been fighting with Barnes & Noble Inc's Nook and Apple Inc's iPad for e-book sales. The Kindle was launched in 2007 and is by far the best-selling device made specifically for reading digital books.

Barnes & Noble is holding a press event Tuesday in New York to unveil a new e-reader. Last month, Barnes & Noble introduced improvements to its Nook Color.