Here's how the App Store bills the Google Books app:
The Google Books app offers access to over 2 million Google eBooks on your iPhone, iPod touch, or iPad. Take your favorite books with you on the go.
Great Reading Experience
- Change font, search within book, information about book
- Night-reading mode
- Offline reading mode
Discover Millions of Titles
- Millions of books in every imaginable category, from New York Times bestsellers to favorite classics
- Over 2 million free ebooks available instantly
- Over 400,000 ebooks for purchase
- Free preview pages
- Unlimited storage of books in the digital cloud
- Worry-free archive
Syncs With Your Google Books Library
- Find books at http://books.google.com/ebooks and add them to the 'My Google eBooks' shelf to sync
- Access all your ebooks wirelessly from the digital cloud; no dedicated e-reading device required
- Automatic page position synching across devices -- pick up reading where you left off
All that being said, I like the app, but I don't see giving up using the Kindle app for books and reading. I think the Kindle app on my iPad is the rest of the eBook store apps. But give Google Books a try, it will continue to grow and get better as feedback comes in from users.