My neighbor gave me a great tip: check out anything by Terry Pratchett that I can find in the youth fiction section of the library. My wife and I are reading them to children ages 9 - 16, and all of them are liking the books.
It can easily be read as funny little books following the sagas of recuring (anti)heroes. But the light-hearted stories belie a sharp-eyed critique of society, with the most well known example probably being Vimes' Boot theory of socioeconomic unfairness.
Often, the subtility is hidden in a word or a choice of words that can easily go unnoticed, but are all small hints of the vast pluridisciplinary depth of knowledge of the author. The esoteric, historical, literary and physical sciences references (to name just them), are delightful small winks to the reader sprinkled throughout the books.
