I AM SO EXCITED I NEED TO WRITE IN ALL CAPS!!! Seriously, this is going to be the best thing ever. While you're signed into your Google Account, just search for google library or something like that. So far, you can search for books and add them to your library, add by ISBN, rate your books, and share your personal library with others.
But what I'm *really* excited for are the not-yet-released features! I mean, I would love to know how my copy of the Counterinsurgency Field Manual (yes, I just bought it) was influenced by Hammes, John Robb and Thomas Barnett, and now, I have a potential way to do this. Of course, the Library feature is still barebones (though Google Book Search is awesome in it's own regard).