Books are among the last bastions of ad-free content. But they won't be so forever if Amazon has its way.

The online retail giant has been nurturing a growing e-reader market with its Kindle device; analysts estimate more than a million have been sold since its 2007 debut. And the idea of serving ads in e-books has been a subject of chatter for a while. But Amazon appears to have taken the next concrete step in that direction. Recent reports indicate the online retail giant has filed patent applications to stuff digital books with contextual advertising.

Forrester estimates that 13 million U.S. consumers will use some kind of e-reading device by 2013. Meanwhile, Barnes & Noble in March bought e-book seller Fictionwise, and Google is set to sell e-books later this year. Mobile e-reader apps such as Amazon-owned Stanza, with its own 2 million users, are also setting the stage for e-books to mushroom. A Bowker survey in the first quarter found that digital books account for just under 3% of total U.S. book sales, up from 0.4% for all of 2008. Books rang up $24 billion in sales last year, according to Association of American Publishers estimates.

 Amazon may be the biggest player in the burgeoning e-book market, but it's not the only one; Sony and upstarts such as the U.K.-based Plastic Logic have entries. Not to be outdone, Apple is rumored to be working on a tablet device.