My son and I have been reading the old Hardy Boys novels. (My wife and daughter are reading Sword and the Stone). The vocabulary is great. Every few pages there are useful words to challenge him. The plots aren't bad either and it develops his sleuthing talents, which is a plus. I just hope he's missing the fact that every time a woman does anything, she's described by her appearance:
"Any luck sleuthing?" Pretty, brown-eyed Callie Shaw asked the Hardys."
-- from The Secret of the Old Mill.