If I had known this book wasn't your typical contemporary realistic middle grade book that's coming of age and sweet and probably also a bit sad, I would have read this much sooner. It has an element of another genre in it that I was totally in the dark about. Don't let the cover fool you-- it may look cartoony and very typical middle grade, as if it would fit right in with Humphrey books (and I love Humphrey books, but...), but this is an upper middle grade book with all of that and a lot more going on. I'd say fifth and up for the most comfortable fit, though my 10 year old just finished up fourth and enjoyed it. (Also, after reading the book, the cover is full of tons of meaning...so it's like a secret message, which is also neat.) 
 I don't want to spoil anything, but I'll say: just go read it. We actually listened to the audio book version in the car and it was fantastic. Also, one side note, speaking of spoilers-- if you haven't read A Wrinkle in Time and want to, read it before you read this if you hate spoilers. The main character of When You Reach Me loves A Wrinkle in Time and gives away tons of parts of the book while speaking to another character. In a way, I am hoping it will help my daughters decide to read it, since they have refused...maybe now they will be intrigued enough to stick with it. I love L'Engle and her thoughtful story and I love THIS thoughtful story. It made me ponder and guess and wonder and worry and hope-- all the things a great book does. My husband, myself and both my daughters (10 and 12) loved it. I think it would have been hard for my 10 year old (who reads well) to read it alone because it bounces around in time. With the audio book in the car tip, we could pause it and discuss when anyone got confused or wanted to discuss theories. LOVED it.