If you use the Kindle App on your Nook and are having problems with your display dimming only when you use that app, then here is the solution to that problem!
The display brightness for the Kindle App is located in the Aa menu drop-down. Tapping the Aa icon in the menu locatated at the top right of the screen will bring up a list where you can change the font size and other settings--one of those being screen brightness! At the very top is a slider control, but if you just want it to be the same brightness setting that you use when reading Nook books then just check the box "Use system settings" . That's it, done! Now switching between reading a Nook book and a Kindle book won't change your brightness settings!