How to Buy Books from the Google Play Store
Click the thumbnail of the book you want to select.
Feel free to browse! (Please note that the choice shown is for the earlier versions of the Galaxy S line!)
When you are ready to purchase, tap the blue box with the price. If you have a credit card on file with Google through your Gmail account, it verifies that you want to buy.
Tap Buy, and the book is on the way to the address you have saved!