When you visit books.google.com, you'll be presented with the option to search Google Books or to browse through the Google eBookstore.
Google Books strives to provide information about print books in a way that's accessible to digital users for free. This includes bibliographic information, some limited previews, and full view of public domain works.
Google eBooks are made available for sale by their publishers. Just as with Google Books, you can preview a limited number of pages. However, if you like what you see, you have the option to purchase a digital copy of the work, which you can then view in full on most Internet-enabled devices, or even download.
When you're viewing a Google Book, you'll easily see if it's available for purchase as a Google eBook.