It looks like the iPad has failed to dent another market: Amazon this week announced that Kindle unit sales accelerated each month in the second quarter of 2010, and that this was true both on a sequential month-over-month basis and on a year-over-year basis. The message is clear: During the time period in which Apple began selling its iPad tablet device, Kindle sales have only accelerated.

Amazon was able to keep up this seemingly torrid pace thanks to a price cut in June, when the Kindle fell from $260 to $190. "We've reached a tipping point with the new price of Kindle," Amazon CEO Jeff Bezos said. "The growth rate of Kindle device unit sales has tripled since we lowered the price. In addition, even while our hardcover sales continue to grow, the Kindle format has now overtaken the hardcover format. Amazon.com customers now purchase more Kindle books than hardcover books—astonishing when you consider that we've been selling hardcover books for 15 years and Kindle books for 33 months."

Amazon remains cagy about actual Kindle device sales and has yet to provide any concrete numbers to back up its claims. However, there are some clues in this week's announcement. The company said that "millions" of people are using Kindle devices, and that author James Patterson had sold 1.14 million e-books through July 6; of those, 867,881 were Kindle books.

Amazon provided some other interesting statistics. Kindle books are now outselling hardcover books by almost two to one, thanks to a sharp increase in the past few months. The company has sold three times as many Kindle books in the first half of 2010 as it did in the first half of 2009. And Kindle's book-sale growth rate has exceeded the industry growth rate for book sales on all eBook readers combined. Five authors—Charlaine Harris, Stieg Larsson, Stephenie Meyer, James Patterson, and Nora Roberts—have each sold more than 500,000 Kindle books.

All this information is designed to counter ongoing news of Apple's success in selling the iPad, which includes, among other things, a software eBook reader. Even though they lack a color screen, Amazon's Kindles offer a superior book-reading experience thanks to their non-glare, paper-like surfaces, much lighter weight, dramatically better battery life, and much lower device pricing. The Kindle is also backed by a much richer library of content, with 630,000 books (510,000 of which are $9.99 or less, lower than the $14.99 average price for Apple's eBooks). Apple offers only "tens of thousands" of titles in its iBooks store.

Apple, of course, has a lot invested in the iPad. So much so that it issues a press release every time it sells a million units. The company will announce its second-quarter financial results on Wednesday, so we can expect an iPad sales update at that time. It should be north of 4 million units total, based on previous announcements.