Ah jimminy....Amazon has you pretty well locked up...my Fire 7 finally failed...replaced the battery and then it had charging problems, I suspect the itty bitty power connector was hosered after so ...anyway, bought a new one because I had downloaded so many books over the years from Amazon and B&N...200+ and didn't want to lose them..
It came in two days via Prime and then I needed to get all my books back...no problem on the Amazon side as everything I bought was stored in the Cloud and downloaded automatically...the Nook books were another problem..
To make a long story short, it took me about 2 hours to do it...the Fire does NOT come with a NOOK reader, or Google Play to get a NOOK reader (of course Amazon app store doesn't have one)...so off I go to ask.com to find pages to explain...
Have to turn on 'sideloading' on the FIre,(via Security & Privacy) then download 4 separate file that mess with the Device Manager and some other stuff.
That loads Google Play...then go there, sign in with your Google account, find the NOOK reader and load and install it. Then use it to log into your NOOK account and wowser, ALL the books download again with one click..so all is well finally....