According to the changelog the new app adds a way to group ebooks by series. It also reportedly adds an option to log in with your Google+ account.
While the former feature is going to be useful, the latter feature was already present in the app. I checked before I updated, and Google+ was listed as a way you could sign in (assuming you're one of the hundreds of people with an active account).
If you see another unannounced feature after you update your app, please leave a comment.
- Convenient group by Series option in your Library.
- New Google+ sign-in.
- Bug fixes and general improvements.