Since different editions of A Northern Light will vary in pages, perhaps pointing to the chapter will be of assistance. It is in Chapter 32 that Mattie learns from one of Grace Brown's letters that she is unable to swim; this information is crucial because in Chapter 28 Mattie finds a letter in which Chester Gillette wrote to Grace about swimming, boating, and other water activities, casting doubt upon his having actually drowned as was supposed.
Previously, too, in Chapter 19, Mattie becomes aware of two men talking outside her window and overhears one musing that if the two people drowned, "the bodies" should have been close to each other in the water. Also, in Chapter 24, Mattie reads one of Grace's letters and learns that Grace was pregnant with Chester's child. So, there is a motive for Chester to be rid of Grace as he boasted to her of all his female conquests and never responded to the news of Grace's pregnancy when she informed him. With this knowledge, Mattie truly learns the significance of what she has remarked upon early in the narrative,
What had I seen? Too much. What did I know? Only that knowledge carries a damned high price. Miss Wilcox, my teacher, had taught me so much. Why had she never taught me that?
Mattie leaves the Northern Woods, having decided to reveal the story of Grace Brown to the world.