This was one of my favorite books when I was young. I'm sure I read it five or six hundred times. The main character, Sam, runs away from New York City to live on his family's historic farmland in the Catskills. He has decided to live "off the land," and after doing research on just how to do that (in the library!), he actually does it.
I loved the fact that he hollowed out a tree as a house, made his own clothes out of deer skin, ground up acorns as flour, dug tubers, and generally managed to live a pretty good life in the wilderness.
Of course, the end is kind of sad when various people find him, and then his family comes and moves onto the land also. But I suppose Sam had discovered that humans are social animals after all.