I first read the Pimpernel Gold during middle-school, and then again in college. While it's prose at best is simple enough for a middle-schooler, the story is impossible to put down at any age.
This true story has everything: evil Nazi's, a train full of royal gold, a young officer charged with shipping the gold out of occupied Norway, partisans hiding from the Gestapho, lovers separated by war, and plenty of near-escapes.
If ever there was a WWII story worthy of a movie screen-play, this is it!