The new look is a clean and simple UI with the accent color for the Play media section. So, for the Books app, the accent color is the light blue, whereas with Music the accent color is orange. Just like Music, the Books app now has the navigation tray that will pop-up if you tap the icon in the top left. And, the update also adds a confirmation dialog so you don't accidentally remove a downloaded book from your device.
But, the best new feature is that you can now upload your own EPUB or PDF files to the Play Books website, and read them on any device you'd like. You can upload books or import books that you have stores in your Google Drive. Storage is set at 1,000 books as long as each is under 50MB. That's pretty solid.
If you want the update just grab it from the Google Play Store.