I've been watching an outstanding series on the History channel called GANGLAND. It's a brutally honest series about the many, many gangs in America. The show goes into great details about specific gangs, and has incredible footage of gangs in action and interviews with former gang members and police officers. The two best episodes were about the Hell's Angels (I can't believe this even made it to air) and the huge problem the military is currently having with gang members working within the war (including skinheads).
Even here in LA I learned about the Mexican gangs in LA. It's pretty scary.
I highly recommend the series.