I have not read the book, but I have seen the film. So I did a little research for you.

This book review from "The New York Times' provides a nice overview:

http://query.nytimes.com/gst/fullpage.html?res=9504E0DF123FF930A3575BC0A9659C8B63

"And even as Amir grows to manhood, settling comfortably into America and a happy marriage, his past shame continues to haunt him. He worries about Hassan and wonders what has happened to him back in Afghanistan..."

These bloggers have written in-depth overviews of the book's major themes:

http://blogs.nlb.gov.sg/highbrowseonline/whos-reading-what/sharon-thien-has-read-kite-runner/

"The book is about friendship, betrayal, forgiveness, redemption and healing on a very personal level - reflected in how Amir finally rescued Hassan’s son Sohrab from a horrific fate and in doing so, found ” a way to be good again” like what Rahman has predicted. Somehow, I wondered if the themes carried to the next level to Afghanistan - to forgive the mindless devastation wrought by misplaced ideologies and to find redemptive means and hope to heal the land and its people."

http://boudreauxjacques.blogspot.com/2008/12/can-we-learn-from-culture-so.html

"Amir embraces the ideals of the American life of freedom, but Baba stays hopeful of return to his old life in Afghanistan. As time passes Amir comes upon an opportunity to redeem himself from his guilt by saving the late Hassan’s son, Sohrab, from being a sex slave. Though no easy task, Amir becomes successful. We witness the transformation of a guilt plagued selfish boy into a redeemed and unselfish man."

Finally, I'd recommend definitely reading through this blog post, which is written ona very basic level but seemed to be an original piece of writing about the book and is very thorough on this subject in particular:

http://essay001.blogspot.com/2007/11/kite-runner-amirhassans-relationship.html

"As young boys becoming adolescents, Amir and Hassan enjoyed doing everything together. However, Amir never considered Hassan and him friends. Amir felt this way because he knew that neither history nor religion changed who they were. In the end, Amir was a Pashtun and Hassan was a Hazara."