This is the book that I was thinking of when I put up the My Side of the Mountain Post though I love both books, just got them mixed up. "The Sign of the Beaver" by Elizabeth George Speare is an awesome book. It is a children's book, but I love it.
http://www.christianbook.com/Christian/Books/product?item_no=338905&netp_id=221496&event=ESRCN&item_code=WW&view=covers (http://www.christianbook.com/Christian/Books/product?item_no=338905&netp_id=221496&event=ESRCN&item_code=WW&view=covers)