Definitely Hatchet and My Side of the Mountain for "young adult" fiction. ADD's right about Paulsen's non-fiction too- very enjoyable.
Off the top of my head, On the Beach by Nevil Shute is a great, and somewhat different, sci-fi apolocalypse work. I remember really liking Lucifer's Hammer by Niven and Pournelle (?) as well.
For non-fiction, the stuff by Tom Brown can be interesting. If you can find it, No Surrender by Hiroo Onoda is interesting- a two Japanese soldiers in the Phillippines don't find out WWII has ended until thirty years later. I don't know if it's still in print- I found my copy used years ago.
Lastly, as a tremendous Cormack McCarthy fan, I have to recommend The Road as well. If you like apocalyptic stuff it is outstanding.