http://books.google.com/googlebooks/agreement/#3Three types of books
This agreement helps define how our users may access different categories of books on Google Books.
In-copyright and in-print books
In-print books are books that publishers are still actively selling, the ones you see at most bookstores. This agreement expands the online marketplace for in-print books by letting authors and publishers turn on the "preview" and "purchase" models that make their titles more easily available through Book Search.
In-copyright but out-of-print books
Out-of-print books aren’t actively being published or sold, so the only way to procure one is to track it down in a library or used bookstore. When this agreement is approved, every out-of-print book that we digitize will become available online for preview and purchase, unless its author or publisher chooses to "turn off" that title. We believe it will be a tremendous boon to the publishing industry to enable authors and publishers to earn money from volumes they might have thought were gone forever from the marketplace.
Out-of-copyright books
This agreement doesn't affect how we display out-of-copyright books; we will continue to allow Book Search users to read, download and print these titles, just as we do today.