The Kite Runner by Khaled Hosseini (a very good novel about many things, including friendship)
American Sympathy: Men, Friendship, and Literature in the New Nation by Caleb Crain (a scholarly study of the subject)
“American Sympathy is a brilliant book about the essential issues and authors of early American literature, and it is written with a slyness and seductiveness worthy of those issues and authors. Indeed, it is one of those rare books that change with way we think about things that matter. Either as an indispensable text or as a cult classic, it will endure.” --Michael Zuckerman, Professor of History, University of Pennsylvania