The title—A Northern Light—gives us far too little guidance about the book. The words could mean so many different things. "Light" implies an inspiration, a hope, or a dream; think a light at the end of a tunnel or a light in the darkness. And "northern" indicates the significance of the setting, which is the North Woods, or the Adirondack Mountains in upstate New York. But "northern" also implies the importance of directionality, like a compass. And if we think about Mattie, she's continually searching for direction in her life.
And then we have to think about who or what "a northern light" refers to. Is it Mattie? Is it Grace Brown? Is it the dreams of both women? We never really know.
So the title's completely up to interpretation, a fact that's further complicated by the fact that the U.K. title of the book is A Gathering Light. Now ponder that.