Tiny Infinities by J. H. Diehl

After fleeing her house while her father moves out, twelve-year-old Alice finds a little girl named Piper and saves her from being hit by a car. Upon their return, Piper's mother sees that Piper, who hasn't spoken since she was two and seems not to be able to hear either, responds well to Alice. That's how Alice gets a summer job as Piper's mother's helper. Alice also finds a new friend in Harriet, a quirky, hyperintelligent girl new to her swim team, and in Owen, Piper's older brother, both of whom help Alice tackle the mystery of Piper's as yet undiagnosed condition. While the innovative situations keep this plot moving, it's the character portrayals that shine. Alice starts off righteously angry at her adults and stays angry, figuring out ways to avoid confrontations but not shying away from them when they're unavoidable. Harriet and Owen seem at first to be merely distractions for Alice but ultimately mirror her discontent with adults who can't see their kids through their own haze of expectations and assumptions. All of this personal growth is seamlessly and subtly woven into a story that is as smoothly lyrical and full of ordinary wonder as it is realistic.