Almost invisibly Amazon has boosted revenue nearly 30% this year to date, to $5.5 billion, behind a smaller, lighter, less expensive $139 Kindle e-reader and its store of more than 720,000 books -- including nearly 600,000 selling for less than $10 and not including 1.8 million free public-domain volumes. Never mind Amazon's segmented sales categories such as Amazon Student and Amazon Mom that offer promotions, discounts and Amazon Prime free two-day shipping. Pay no attention to Amazon Web Services as it drops prices, adds usage tiers and incorporates tools from Oracle ( ORCL) to help developers launch apps. The key to Amazon's continued success is constant upgrades and tweaks to its Kindle offerings, including the launch of two in-house publishing imprints and the cross-platform growth of Kindle apps for Apple, Research in Motion's BlackBerry and Google Android products. Amazon realized well before announcing it this summer that e-books were going to outsell hardcovers, but now that the gap has grown to nearly 2-to-1, it takes a clever company to increase its advantage in a digital media sector facing tough competition from Google and Apple.