From the announcement:
These days, 3D seems to be everywhere. From Hollywood blockbusters to LCD televisions, those iconic cardboard glasses seem to be everywhere, even on President Obama. Our friends at YouTube released a 3D experiment, and NASA released 3D images from the Mars rovers.This got a few of us on the Google Books team to thinking: why not offer our books in 3D? So we've done exactly that. Now, when you visit a book, you'll see a new option: "View in 3D".