The genre of Jennifer Donnelly's A Northern Light is "Young Adult fiction." These books are also called "problem novels" or "coming of age" novels because they examine the difficulties teenagers often face as they transition from childhood to adulthood.
Characteristics of these kinds of stories include a concentration on "plot, setting and character," elements that more readily engage readers of this age group, rather than the elements of traditional fiction such as...
In essence, the character and plot are of paramount importance as they allow the reader to more readily identify with the problems being faced by the main characters. The setting supports the character and plot. Another characteristic of these books is that the main character, or the protagonist, is a teenager rather than an adult.
The subject matter and story lines are typically consistent with the age and experience of the main character, but beyond that YA stories span the entire spectrum of fiction genres.
Certainly we can see these aspects of the genre in the story of Mattie Gokey. She is a young girl with dreams that, if realized, will take her out of tedious and predictable life she would lead at Big Moose Lake in the Adirondacks. Mattie wants to go to college to be a writer; her father and Royal (the young man who likes her) expect that she will stay where she was born and build a life as her parents did before her. Mattie cannot see any happiness in her life if she does this.
However, this is not her only difficulty: Mattie has letters that belonged to a young woman found "drowned" in the lake. These letters hold the clues to young Grace Brown's true fate. While she had asked Mattie to burn them, Mattie struggles with this promise, believing she must expose the truth they reveal—thereby exposing her murderer, but breaking her promise to Grace.
This is a book about change, something central to life, and particularly noticeable by teenagers—for the world won't stop even if they want it to. Mattie feels and fears this change, when she
...will never again be quite the same person...
Mattie also realizes that the world of fiction is not necessarily reflective of the real world. Certainly the truth of Grace's death has taught her this. Mattie reflects:
People in books are good and noble and unselfish, and people aren't that way...and I feel, well, hornswoggled sometimes. By Jane Austen and Charles Dickens and Louisa May Alcott. Why do writers make things sugary when life isn't that way?
A Northern Light is an exemplum of a coming of age novel, with a main character that struggles with change and knowledge of the world, in much the same way as teenagers the world over, for thousands of years.