I really have no idea, but I did a little searching online and found this article about Donald Hall. From the date that he moved to New Hampshire being 1975 and what it says about that – and then the book was published in 1979, I would tend to guess NH as the setting. But like I said, just a guess. I also saw a link to a Reading Rainbow episode about this book where they visit a place in Massachusetts, so who knows! Just thought I’d throw it out there. We really liked that book.
http://www.library.unh.edu/special/index.php/exhibits/jane-kenyon-and-donald-hall