SAN FRANCISCO (Dow Jones)-Ecommerce giant Amazon.com Inc. (AMZN) has acquired a small startup that makes a free e-book application for Apple Inc.'s (AAPL) popular iPhone.

The acquisition, which Lexcycle Inc. announced Monday on its blog, appears to be an attempt by Amazon to strengthen its presence across an array of mobile platforms beyond its Kindle ebook reader.

Lexcycle's Stanza application enables iPhone users to download content from a selection of over 100,000 books and periodicals in ePub format, an open standard for e-books that Amazon's Kindle currently does not support.

"It's very early days for ebooks and we believe there is a lot of innovation ahead of us," said an Amazon spokeswoman. "Lexcycle is a smart, innovative company and we look forward to working with them to innovate on behalf of readers."

It was not clear how much Amazon paid for Lexcycle, a year-old company with offices in Austin, Texas and Portland, Oregon. The company said more than one million iPhone and iPod Touch users had downloaded Stanza at the end of 2008.

Shares in Amazon closed down 1.6% at $83.12.

Amazon last week said Kindle sales continued to exceed expectations, but the Seattle, Wash.-based company once again declined to provide specific sales figures.

The e-commerce giant offers more than 270,000 books, as well as top newspapers and magazines, for sale on its Web site. The company last quarter also launched a free application that allows customers to enjoy Kindle titles on the iPhone and iPod touch.

"We are not planning any changes in the Stanza application or user experience as a result of the acquisition. Customers will still be able to browse, buy, and read ebooks from our many content partners," said an entry on Lexcycle's blog.

-By Scott Morrison; Dow Jones Newswires; 415-765-6118; scott.morrison@dowjones.com