In A Northern Light, why do you think Grace asks Mattie to destroy her letters?
While Mattie is working as a maid for the Glendale Hotel she is asked by a guest, Grace, to burn some letters. Mattie puts the letters into her apron pocket and doesn't give them another thought until the woman turns up dead. She had gone out on the lake with a male friend who turns out not to be who he said he was.
As the reader learns Grace is pregnant and the letters are love letters. During this time in our history it was not acceptable for a lady to act as Grace has acted and to be unwed and pregnant was something "good women" just didn't do. Grace asked Maggie to burn the letters so that no one would find out about her secret and maybe because she knew that she was in danger. Grace was in a bad position and I think she knew that she would probably not return from the afternoon outing.