A Wind in the Door (Time Quintet, Bk. 2)

Meg Murry can't help but be worried when her six-year-old brother, Charles Wallace, announces there are dragons in the vegetable garden. He's so bright, and so different from other kids, he's getting bullied at school, and he is also strangely, seriously ill. But Charles Wallace is right about the dragons - actually a friendly entity who has come to help Charles Wallace fight his sickness, and to take Meg and her friend Calvin O'Keefe on a terrifying, wonderful journey into galactic space - where they must battle the force of evil to save Charles Wallace, and themselves.