The reply is correct. With the Ipad you need an app for each seller. If you have an account with B&N you need the Nook (free) app, if you have an Amazon account you need the Kindle app. To use the Ibooks app you need to buy from the Ibooks store or download pdfs. There are other erading apps also, th Kobe from Borders, Stanza which is nice for Smashbooks books and othere and there are a few more. They aren't interchangeable. All the apps are quite nice though and you won't be disappointed with them. i expect once yiu have the Nook app installed you will need to go back and download your book from the archives in your account.
Diane B