Malcolm Klein, author of several books on gangs and delinquency going back to the early 1960s. A must read book of his is The American Street Gang (1995) where he makes an interesting arguement by linking William Julius Wilson's "Underclass" theory of the ghetto to gang formation. John Hagedorn made a simialar argument in 1988 with People and the Folks: Gangs, Crime and the Underclass in a Rustbelt City, but Klein's analysis has a stronger conceptual backing.

He just recently retired as head of the Social Science Research Institute at the University of Southern California. He is the editor of The Eurogang Paradox : Street Gangs and Youth Groups in the U.S. and Europe which came out in 2000. And if you are looking for a good text book to teach a class on gangs The Modern Gang Reader that is co-edited by Klein is very usefull.