Google Books has officially been around since 2005 and has evolved over time. Despite some early controversies the service offers many digital resources in a handy user and search interface. Some new features worth checking out are highlighted in this video by Richard Byrne and his blog post.
For more about how to use Google Books, check out this Google support page.
For a few other sites with free digital resources, check out Project Gutenberg, Europeana, and the Internet Archive.