Being less than five years out of college, I would highly recommend "A Sand County Almanac," and "Desert Solitaire." I also enjoyed "A Walk in the Woods," by Bill Bryson. I'm working my way through "Cadillac Desert" right now and find it very dense; I doubt it would hold the attention of many college freshmen. I think books by Michael Pollan and John McPhee are well written and would be able to hold a 17 or 18 year-old's attention.
Cheers, Erin Brandy
