In school we are studying hip hop and what race has to do with it. Wanted to know how the word came to enter the genera and become so mainstream. New York New York by Grandmaster Flash is what i found to be the first instance in hip hop to use nigger (well nigga).
If you want to get an audio history you should look at spoken word artists/poets from the 1960s thru 1970s who've directly influenced hip hop. Many of the poets used the word nigga in a positive, negative, sarcastic, playful and social critique kind of way. Some accessible artists/albums that highlight this are:
Kain - The Blue Guerrilla
Bama The Village Poet
The Last Poets
Lightnin' Rod - Hustlers Convention
And if you can find this album give it a listen:
Margaret Mead & James Baldwin - A Rap On Race