OMG I got all the way through the thread and nobody mentioned Franz Fanon? Okay, heavy reading. Even heavier is Gayatri Chakravorty Spivak, and her famous (but nigh impenetrable) essay, "Can the Subaltern Speak?"

Seconded the nomination of Said, whom I find much more readable (and I always liked him when he popped up on PBS). Ngugi is occasionally difficult for me, but, I like his trucculence. (The book of his I read was Moving the Centre.)