Graffiti anyone?

Before school was over, I polled my 6th grade students about what they wanted to do next year in 7th art.  One of the first things many of them suggested was graffiti art.  I have decided that my first project with them will be to create a tag to design the cover of their art journal.  Does anyone have a really great power point they'd be willing to share with me about the history of graffiti...or a site that has good handouts/information about graffiti?

 I purchased the class pack of 30 6"x9" journals from Dick Blick.  The covers are cardboard and they are spiral bound.  I plan on having the students design the front cover with a tag of their name.  They'll be able to use tissue paper, paint and I'm thinking the Crayola Airbrush Marker Kits I have (which would make it even more authentic...closest thing to spray paint!).

I have a very vague knowledge about the history of graffiti.  I know that prehistoric art can be considered graffiti, and that graffiti dates way back in history to the Egyptians and Greeks.  I also know of the political reasons behind graffiti in the 60's and 70's and that Keith Haring was a subway graffiti artist during a point in his life, but I need some good vocabulary and some reading information about the topic to make a good, solid unit.  I'd even be willing to purchase a book about graffiti if someone highly recommends one!  I currently have a $50 gift card to Blick that I could spend and there is free shipping on orders of $45 or more that expires at midnight tonight, so any feedback would be great, especially if Blick has a book that would be good to order!  Thanks in advance!