Amazon.com CEO Jeff Bezos spoke today at Wired magazine's Disruptive By Design business conference, revealing how successful the Kindle -- in all its forms -- has been during its 18 months on the market.

According to Bezos, 35 percent of the sales of the 300,000 books available for the Kindle have been sold in electronic form. "Internally, we are startled by that statistic," he said.

Bezos was also very optimistic about the Kindle's success moving forward. He seemed certain the device will eventually replace physical books.

"Reading is an important enough activity that it needs a purpose built device," he said. "Some people ask me, 'Why would I buy this device, it only does one thing?' That's exactly why you should buy it." Then Bezos revealed that he gets "grumpy" whenever he's forced to read a real book. "I didn't realize all the failings of a physical book," he said. "[Books have] had a great 500 year run. It's an unbelievably successful technology, but it's time to change."