From the Google Apps Updates Blog:
EPUB is the most widely supported file format for publishing accessible documents and digital books, allowing people to consume long-form content in their preferred apps on a diverse range of screen sizes and devices, including smartphones, tablets, and eReaders. To make it easier for authors, publishers, and academic institutions to create works that can be consumed digitally, today we’re launching the ability to export Google Docs files as EPUB publications. Simply go to the File menu > Download as > EPUB Publication (.epub).
See Also: List of All Google Docs Export Formats