Kristy and the Secret of Susan — Ann M. Martin

Kristy's newest baby-sitting charge is Susan Felder, who goes away to a special school. Susan isn't like most kids. While she can play the piano and sing beautifully...she can't talk to anyone. Susan is autistic. She lives locked inside her own secret world.

Kristy thinks it's unfair that Susan has to be sent off to school and is treated differently from everyone else. But Kristy's going to try to change that -- by showing everyone that Susan's a "regular" kid, too. And then maybe Kristy's new friend can stay in Stoneybrook for good.