When You Reach Me by Rebecca Stead. I was first introduced to this book by my 11-year-old niece: "It's my favorite book. You have to read it."
the protagonist sixth-grader, is likeable as she rambles around New
York City in the late 1970s. She lives with her quirky single mother, who is obsessed with going on
the TV game show $20,000 Pyramid. Miranda's life gets complicated when her best friend, Sam, inexplicably pushes her away and she begins to receive mysterious notes with messages that
are important to the future, but have no context in the present, such as
"I'm coming to save your friend's life, and my own." The story circles
back on itself and you may end up wanting to re-read it to make sure
everything holds up.
Even though it's a mystery with a sci-fi time-travel bent, it's a quite realistic read overall and uses the time-travel
angle poignantly. If you're a fan of L'Engle's A Wrinkle in Time, you'll enjoy this one, and if you need further convincing, When You Reach Me won
the 2010 Newbery Medal.