Quite frankly, I'm not postive when this capability appeared in Google Book Search. I was checking some of my Google Book Search books which I have saved as favorites, and I am now able to scroll through the entire contents of each book. If it stays around, this is a huge new feature. Here are some examples:
I need to spend more time with Google Book Search. Here is an earlier screencast / tutorial I've created on the topic.
Stephen Downes correctly noted in this post's comment, that it's only an expanded preview. Drat! Still, the expanded preview is nice, and one can normally search within a book to pull up specific pages.