Welcome to the forum, Kent.
This is a problem - most books purchased from Amazon will have their Digital Rights Management (DRM) applied, which prevents copying, but also use outside a Kindle application. B&N use their own, different, DRM.
Some people strip the DRM from their files, allowing them to change the format, and protecting them from future problems. Others are uncomfortable with this due to technical issues or legal ones.
Most people agree that it's a pain!
Amazon have a Kindle application available on many platforms, so that will allow you to read Kindle content on an iPad/iPod/iPhone, Android phone, PC and Mac.