The Adventures of Huckleberry Finn is the quintessential American novel. It works on several levels: boy adventure story, social commentary, and coming-of-age character study. It's written in the voice of Huck Finn, and although it takes a while to get used to the various dialects, once you get the hang of it, it feels completely absorbing and authentic.

Short shameful confession: when assigned this in high school, I read a massively abridged version (aced the test though, baby!). I'm glad I finally got around to reading the real thing. At any rate, it's a different (and better) novel now than it would have been at age 16.