Well, I'll tell ya. What they say here is this:
Many of the books you can preview on Google Books are still in copyright, and are displayed with the permission of publishers and authors. You can browse these "limited preview" titles just as you would in a bookstore, but you won't be able to see more pages than the copyright holder has made available.
But wait. "Just as I would in a bookstore?" That's actually not true. I could read the whole damned book in the bookstore if I wanted.