The nook color spec: https://nookdeveloper.barnesandnoble.com/product/nook-color-specs.html
Contains the android.os.Build as mentioned by CommonsWare.
And the same for Kindle: https://developer.amazon.com/sdk/fire/specifications.html
Anyhow, it seems better to use the android.os.Build.MANUFACTURER. It will be 'Amazon' or not.
And its a duplicate of: Detect app is running on Kindle Fire