The battle between Sony's and Amazon's e-book readers is heating up now that Google is stepping in to throw some new kindling on the fire.

The search leader said it plans to make half a million of its digital books available for free on Sony's Reader devices -- a potential blow to Amazon, which has created a brisk business around its own e-reader, the Kindle.

The move marks the first time Google has made its collection of scanned, public domain books available and optimized for an e-reader. It also means Sony's available digital library now numbers around 600,000 titles, which puts it above Amazon's Kindle-compatible library of about 245,000 titles.

For Sony, the Google announcement signals that the electronics giant is not content to stay in second place and is stepping up to take on Kindle. While the recent launch of Amazon's second-generation Kindle 2 grabbed headlines -- CEO Jeff Bezos even went on the talk show circuit to tout it -- Sony has quietly been selling two versions of its Reader since 2006.

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