How do you determine if a book is in the public domain and therefore out of copyright?

Whether a book is in the public domain can be a complex legal determination. For users in the U.S., Google Books currently treats all books published after 1923 as protected by copyright, except for books to which no copyright was ever attached, such as books authored by the U.S. government. For users outside the U.S., we make determinations based on appropriate local laws. As with all of our decisions related to the Google Books content, we're conservative in our reading of both copyright law and the known facts surrounding a particular book. If we don't know for sure that a book is in the public domain, you'll see at most bibliographic information about the book and a few short snippets — sentences of your search term in context.