The official Google Blog states that Google Books' public domain books can now be downloaded as EPUB format as well. What makes this significant? Don't we already have pdf?
The reason it's significant is because the EPUB format (unlike pdf format) is designed with 'reflowable digital books and publications' in mind. This means that the text and structure of the book can be easily reformatted to fit whatever device you have in mind unlike PDF which is fairly static. The EPUB format is also open source and so improvements and changes can be made by anyone so you don't have to pay anyone to use the format.
By switching to this format, Google book's library even more attractive to people who don't necessarily have access to a large screen all the time. Now they can read those books on their mobile phone or PDAs knowing they will be properly formatted.
If you're into more free books, don't forget to check out Project Gutenberg since they also have plenty of public domain books.