Undaunted by the rise of the iPad, Amazon's restless chief kept pushing into the e-book space: He quickly launched a Kindle app for the iPad, a Wi-Fi-only Kindle, and a cheaper, slimmed-down machine too. His Kindle store is now the most popular in the nearly $1 billion e-book business, with millions of people buying titles from Amazon's vast book catalogue. But not everything in life can go digital. In November he announced he'd spend $500 million to buy Diapers.com. Stock this year is at an all-time high.