On April 1st, Google enabled 3D viewing on Google Books, the feature was taken down on April 2nd in order to focus efforts on a 4D version. That effort failed miserably, but Google’ve now “enabled the 3D version of Google Books for viewing via a special URL parameter. To see any book in 3D, just add &edge=3d to the book’s URL. Note: be sure to add this parameter before the # in the URL.
More Info: Example URL