A Wrinkle in Time by Madeline L'Engle. It's a book that several of the characters carry around with them, so it would be helpful if you were familiar with it. Also, it's a wonderful story.
When You Reach Me surprised me. It was not at all what I expected before, or even after I had started reading it. If you pick up this book, you must finish it, or you'll never understand what I'm saying. On the face, it's a story of 6th-grader Miranda, going to school in New York City and trying to figure out why her best friend Sal has started to ignore her. Then it becomes a story about why some random kid punched Sal in the stomach. Then it's a story about why there's a homeless guy sleeping under the mailbox in front of Miranda's apartment building, and why he keeps kicking the air and saying weird things. I promise, it will all come together, but you have to read the whole book. It's not a long book, but it is powerful. Every word counts. There are clues, but I couldn't solve the mystery on my own. That's okay, because the author, as they always seem to do, solved it for me at the end.
This is a mystery, but it's also a school story, a friendship book, and so much more. It defies easy explanation, so just (You know what I'm going to say, right?) read it.