Ever since Amazon released the news Monday that it was now selling more e-books than hardcovers, the industry has been examining the veracity of the claim. Amazon, after all, has a tradition of releasing selected figures while withholding total sales numbers, just as it did in this case by not releasing actual sales for e-books, hardcovers or Kindles. Some suggested that Amazon’s e-book to hardcover sales ratio was only for customers who shop in the Kindle store, not for all customers. But Amazon was adamant the figures included all customers of its U.S. book business, and the e-tailer’s claim has been supported by publishers.

Interviews with several major trade houses found all acknowledging that they were selling at least as many e-books as hardcovers through Amazon with one major publisher reporting that in the last couple of weeks the ratio had been higher than the 143 e-books to 100 hardcovers Amazon reported for the second quarter. “[E-book] sales are growing week by week,” this publisher said.

Explanations for the e-book explosion at Amazon revolve around its huge e-book market share compared to its much lower share of the hardcover market. Another publisher pointed to the decision to abandon the windowing strategy in which e-books would be delayed by three months after release of the hardcover. Releasing the much cheaper e-book simultaneously with the hardcover contributed to the shift in sales, this publisher believes. And another publisher noted that Amazon’s e-book sales are not only for titles sold through the Kindle, but for all devices for which apps are available, including Apple’s family of reading devices