While sifting through my book collection for some books to donate to a charity, I found two that I can highly recommend, one for both for adults and kids and the other more for those of us adults who passed through the 70's or 80's when PCs were starting to bloom.
Code by Charles Petzold is easy to read and very entertaining, and contains a very well-plotted development of how computer programming developed over time, starting with codes and combinations, moving into binary then onto logic and switches... flip-flops, bytes, hex, assembly language all develop into microprocessors and on... the book was published in 2000 so it's not that dated. I'm re-reading it now and am glad I didn't let go of this one. Kids and adults will enjoy the simple way the book progresses and believe me - you'll learn something. This book has some very unusual ways to teach some of these topics, but you'll understand them!
The other book, which I'm certain many of you have probably read, is Hackers by Steven Levy. It's not about that element of society that likes to create viruses and malware and other mischief, but about how the computer age sort of started - back at MIT and the railroad club. Trust me, if you've never read this book, hunt a copy down and do so. It's got just a little bit of colorful language, but the history this book provides is amazing - I read this book about once every 5 or 6 years for no reason other than it's just a great book..
I know these aren't NXT-related, but I know a lot of readers are always looking to learn more... these are a good start.