Looks like Google branching out into searching more than just web sites. The Google Print FAQ says they're experimenting with "publications" (books? magazines?):
Google's mission is to provide access to all the world's information and make it universally useful and accessible. It turns out that not all the world's information is already on the Internet, so Google has been experimenting with a number of publishers to test their content online. During this trial, publishers' content is hosted by Google and is ranked in our search results according to the same technology we use to evaluate websites.
Google Print isn't referenced anywhere else on their web site so it's unclear as to whether it's a planned beta, an ongoing effort, or already over, but it sounds like an effort to counter Amazon's full-text book search efforts.
Update: Reader Xavier writes that Google Print is still working. A search for "1,000 knock knock jokes for kids" (with the results restricted to the print.google.com domain) yields this page for the book. A search for a common word like "the" reveals that around 8000 books are available, including Tolkien's Fellowship of the Ring, David Foster Wallace's Infinite Jest, Crime and Punishment, and Kurzweil's The Age of Spiritual Machines.