Betty writes: You talk about the Nook reading app for Kindle. What about a Kindle app for a Nook? Where do you get that from? And if I have two devices in my house (a tablet and a nook) and I want the app on both does it download the same thing on both?
Betty, getting the Kindle reading app for your Nook is super-simple. In May, Barnes & Noble added the Google Play store to the Nook. You can read about that here. 
You’ll need a Nook tablet, not the e-reader. If you have a Nook tablet, it should have updated to include the Google Play store. The Google Play Icon should be front and center on your tablet.
It looks like a little shopping bag. Tap the icon.
When the store opens, type Kindle for Android in the search box.
Install the app and all you need to do is log in to your Amazon account and start reading.
If your other tablet is an Android, go to the Google Play Store using that device and download the Kindle App. If you have an iPad, get the Kindle App from the iTunes Store. It is also available from the Windows Store if you are using a Windows tablet. No matter where you get it, the app is free and should install in minutes. If you’d like to read your books from your computer as well, there are also versions available for PC or Mac at Amazon.com
When you buy a book from Amazon , it’s actually stored in the cloud. You can download it to multiple devices registered to read. (these devices will need to be registered to your Kindle account and there is a limit of six devices for one account. Kindle books will synch between multiple devices. I have a Kindle and often switch between Kindle, Windows Tablet and Android phone when reading a book and usually the pages match up perfectly to the last page read.