R.J. Palacio's best-selling novel "Wonder" was born in a moment of panic. She was getting milkshakes with her two sons in Brooklyn when she saw a little girl with a severe facial deformity. She rushed her sons away as her three-year-old began to cry. She instantly regretted it, and wished she'd stayed to talk to the girl. That night, she scribbled the first sentence of a novel on a Post-it Note: "My name is Ian. To me, I'm an ordinary kid."

She wrote the novel—the story of a 10-year-old boy with a horrible...