Amazon says that second-quarter sales of electronic books on its Kindle outnumbered sales of hardcover books. And by a lot - for every 100 hardcovers, Amazon sold 143 e-books. From the NYT:

Kindle sales increased each month in the second quarter, the same period that Apple began selling the iPad, and the growth rate tripled after Amazon lowered the price of the Kindle from $259 to $189 in late June, Amazon said. That came after Barnes & Noble dropped the price of its Nook e-reader to $199 from $259.