I Am the Cheese by Robert Cormier is the single most greatest book ever written and beheld by the hands of mortals. I love the story. I love the way the story is set up. I love the ending. Love is an understatement, especially about the ending. The ending is . . . well, it's . . . er, just read the book for yourself. Here's a link to its page on Amazon, BUT LET ME WARN YOU: read the first review and ONLY the first review, because it doesn't give anything away plot-wise. I think stories have the greatest impact when you when you go into with no expectations plot-wise. Otherwise, if you know something is going to happen, as you read, you're constantly thinking, Ugh, when is the good part going to happen, and I think that ruins the magic of storytelling. Well, I've ranted enough. Just get the book. Now. Made me cry at the end.
Note: the book has a rather . . . complex storyline and may be hard to grasp for kids of younger ages.