How to buy books for my Iphone playbook?

I had a previous google play book account from my old sprint service but now that I'm on an AT&T iPhone 4 I can't figure out how to buy books from the play book app I downloaded yesterday. Is there another section of the app that I'm just not seeing that allows me to buy books or am I gonna have to start buying my books from https://play.google.com/....com/store?

Iphones, Ipads, Itouch can read any format of ebook if you d/l the application. So quite frankly you can buy books from Barnes & Nobles and read them on NOOK, you can buy from Amazon.com and read them with Kindle app. For apple and epubs bought on ITunes, personally I use Ibook and love it. ( All those apps are free by the way).

I will try to explain with Ibook and hope that Playbook works the same.
IBOOK, when I open the apps it opens on the book shelf. On top I have multiple squares (Store, Collections, the format I want them to showcase and edit. If I hit store it opens ITunes store and from there I can do search and buy.

Once bought they will show automatically in my library.
I usually prefer to buy from computer and sink my ipad afterwards. I need big screen: P

Hope this help.