what is the secret baba and soraya share and how does amir discover it? what does this say about baba? chapter 13

Soraya cared for Baba as though he were her own father, bathing him, reading to him, cooking for him, and giving him anything else he needed. One day, Amir came home to find Soraya hiding Rahim Khan's notebook under Baba's mattress. Baba admitted that he had coaxed Soraya to read him Amir's stories. Amir knew that Baba appreciated his writing. Amir left the room to cry tears of joy, since he knew Baba disliked seeing him cry.