The other comments are really focusing on Reactor theory texts. While these are very good texts, a more general discussion of nuclear engineering is more appropriate for pre-university reading IMHO. The introductory nuclear engineering class at my university uses:
Fundamentals of Nuclear Science and Engineering by Shultis & Faw http://www.amazon.com/Fundamentals-Nuclear-Science-Engineering-Edition/dp/1420051350
I found this to be a very easy read and think that this book covers a much wider range of nuclear engineering technologies than the other texts that have been referenced. I also highly recommend another text by the same authors for radiation shielding.
Good luck!