Located between the "full screen" and "two pages" icons in the toolbar, the thumbnail button spreads all the pages of a book or magazine out for quick browsing and access. There's also a time-saving contents drop-down that allows for jumping to chapters within books or articles within magazines. Only select chapters are listed, however, so you may have to choose the closest one and navigate backward or forward from there.
Other improvements include the ability to embed a preview of a book on web sites, and an option to read books in plain text. It's worth noting that, as with most things related to Google Books, most of these features are available largely for public domain books and those authorized by Google's partner program.
Hit up the full post for the complete rundown of the new features and—if you use Google Books—let us know what you think.
New Features on Google Books [Google Book Search Blog]