The Amazon Kindle has supported this type of sharing via Facebook and Twitter for a while (but only from Kindle devices, i.e., not from the iOS, Android, HTML5, and other Kindle clients). The Kindle approach also supports sharing of highlighted/annotated excerpts, which does not appear to be the case with Google Books. (Click here for a Kindle => Facebook example, if you’re one of my Facebook friends.)
The Google+ “Share box” is clearly going to have multifaceted synergy with other Google services.
Let’s say you are reading Alice’s Adventures in Wonderland, and you’d like your friends to read it with you (or you have a witty comment to let loose). Simply click on the Google+ Share box on the About the Book page or in a Google Books preview, enter your message, then select which circles you’d like to share details about the book with, and click “share”.
The book cover, description and title linking back to the Google Books About the Book page will appear in your Google+ stream with your message.