Is Apple's iPhone emerging as the primary competitor to Amazon's Kindle electronic book reader?

A New York-based mobile application developer, ScrollMotion, is making a host of best-selling books, including Stephanie Meyer's teen vampire drama "Twilight" and Philip Pullman's philosophical fantasy "The Golden Compass," available as applications for Apple's popular iPhone and iPod touch.

ScrollMotion has reportedly cut deals with a number of major publishing houses and plans to roll out a series of new releases and best sellers. It's one of a growing number of e-book applications popping up for the iPhone -- and moving onto Kindle's turf.

The timing is awkward for Amazon, which ran out of Kindles this holiday season after scoring a major product endorsement from talk show host Oprah Winfrey. The Amazon website give an 8-10 week wait for new Kindles. The online retailer is scheduled to come out with a next-generation Kindle next year, possibly in February.

Wired has more on ScrollMotion, noting that it has signed up publishers Houghton Mifflin, Simon & Schuster, Random House, Hachette and Penguin Group USA.

Other e-book apps for the iPhone include Stanza, eReader, Bookshelf and Classics. The Kindle also faces competition from Sony's e-book reader, a new device called eSlick, and others.

The New York Times today explores the growing visibility of electronic books, with the Kindle leading the way. Amazon doesn't disclose Kindle sales figures, but the Times says estimates range from 260,000 to a million units sold. The story also notes that iPhone applications gave a boost to electronic books this year and "are already starting to generate nearly as many digital book sales as the Sony Reader, though they still trail sales of books in the Kindle format."

I asked Ian Freed, the executive in charge of Kindle, about the iPhone application Stanza in an interview a few months ago, but he dodged the question. The growing ranks of iPhone e-book readers, however, is surely on Amazon's radar.