When You Reach Me is a deeply intertextual book. That means When You Reach Me often refers to other books and depends upon those other books to help it make meaning – kind of like a book having a conversation with other books.
In this case, the book that's most woven into the fabric of When You Reach Me is Madeline L'Engle's young adult fantasy novel A Wrinkle in Time. Stead's' heroine, Miranda, carries a copy of A Wrinkle in Time around with her and closely relates to the protagonist of that novel, Meg.
Well, there's an obvious answer to this, of course, which is that both novels are written for young people, both novels could be shelved in the sci-fi or fantasy section, and both deal with time travel in some form.
We'd like to point out, less obviously perhaps, that both books have really, really big hearts. In A Wrinkle in Time Meg defeats the evil IT with love (When You Reach Me, 43.9-15). In When You Reach Me, Marcus goes back in time to sacrifice himself and save Miranda's friend Sal. Though both books are sci-fi, time travelling books, they are also books that are interested in human emotions.
Well, first off, Miranda's family is not Meg's family. Meg's Mom cooks waffles and pancakes for her, while Miranda's mom is a working single parent who tries her best just to put food on the table (30.2). Meg's dad is being held prisoner on the other side of the universe, while Miranda's dad simply isn't in the picture (9.5-6). When You Reach Me, set in the late '70s New York City, is updated to reflect a newer, more modern kind of family.
Finally, When You Reach Me uses its time travel in a slightly different way than A Wrinkle in Time. We might even say that When You Reach Me is a revision of how time works in A Wrinkle in Time. Stead's novel really wants us to think about time as theorized by Einstein. That is, Einstein argued that time doesn't move in a straight line at all. Time is relative. We see this in the novel as events seem to happen out of sequence all the time. See Miranda's conversation with Marcus in Chapter 14 for a full explanation.