When You Reach Me by Rebecca Stead

Another book Penguin didn't get a chance to read, but it was almost due back to the library and I got sucked in. 
The book is written as if it's a response to mysterious letters received by Miranda, the protagonist.  I love that her favorite book was one of my favorite books when I was Bumblebee's age.  A Wrinkle in Time is referenced several times throughout the story.  It's mostly a coming of age story, but I really enjoyed the mystery (Who was this writing the letters?) even after I began unraveling it.  There were enough milder mysteries to keep me reading, everything was interconnected, it just wasn't spelled out. 
This is a great light read (took me an afternoon) for upper elementary school kids and middle schoolers.  It probably isn't quite deep enough to interest high schoolers.  Adults looking for a quick, light but fun read will enjoy quickly working their way through. 

And a bonus...there is also a reference to a medically indicated carb free diet.  They don't go into details, but it's always fun to recognize something of yourself or close friends in a book. 

I can't think of any content warnings for kids.  Any mention of violence is light.  At some point, the mysterious letter writer tells Miranda that the plan is to save two lives, "her friend" and his own.  There's a mentally questionable individual known as "The Laughing Man".  The mystery itself will either intrigue or frighten young kids.  This would be a good talking over book in 4th-6th grade.  2nd and 3rd graders may enjoy it as well.