You can access the Google eBook Store here. And, read more details here.
Currently it is available in US only. For me (I am in India) it is showing below message.
It looks like you're located outside of the United States. Although you're welcome to read about Google eBooks, please note that Google eBooks are only available for sale to customers in the U.S. at this time.
And, Google official blog says,
Launching Google eBooks is an initial step toward giving you greater access to the vast variety of information and entertainment found in books. Our journey has just begun.
Google eBooks can be read using various devices including Web browsers, Smartphones (Android,iPhone, iPod Touch and ipad) and eReader devices.
Currently, Google eBooks are not compatible with Amazon Kindle devices. Google says they are open to supporting Kindles in the future.
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