When You Reach Me by Rebecca Stead

I just finished this year's Newbery Medal winner and now I'd like to reread A Wrinkle in Time, by Madeline L’ Engle. A Wrinkle in Time was also a Newbery Medal winner back in 1963 and shares the concept of time travel. The main character in When You Reach Me has an immense fondness for A Wrinkle in Time and is carrying the book around and rereading it herself. There may be other similarities. Miranda, the protagonist in When You Reach Me has received several notes and the anonymous letter writer conveys things that haven't happened yet and knows things that make Miranda uncomfortable and a little afraid. When and how did the person leave the notes? Who is this person?