Who is the talib that Amir meets with to try to get sohrab? And Also why does Rahim Khan write a last letter to Amir and what new information does he reveal?

THe Talib official in charge turns out to be Assef the same guy who rasped Hassan. Rahim Khan’s final letter to Amir answers all the questions Amir has about his father. In it, Rahim says that Hassan was Amir's half brother,

“Amir jan, I know how hard your father was on you when you were growing up. I saw how you suffered and yearned for his affections…But, your father was a man torn between two halves, Amir jan., you and Hassan. He loved you both, but he could not love Hassan the way he longed to, openly and as a father. So he took it out on you instead- Amir, the socially legitimate half, the half that represented the riches he had inherited and the sin-with-impunity privileges that came with them. When he saw you, he saw himself.” (pg 301).