I have the same problem on my iPhone 6. The app freezes several times during the day, and I've had to reload my entire library several times. Very frustrating.......
I am ruinning IOS8 on an iPad 2 and have had no porblems.
I also have an iPhone6 and will check there for the issue.
I just checked on my iPhone6 running IOS 8.
I signed into the NOOK app, downloaeded and opened my current book, no problem.
I then went into another app and returnd to the NOOK app, my book was still loaded and open.
I then returned to teh home screen and killed the NOOK app. Upon reloading the app, my book was still available on my device.
I have run the same tests that Larry did and I get the same results. No issues encountered, not saying that you are not experiencing these issues but you should call 1.800.THE.BOOK and report then so that NOOK QA can look into them.