I read this before I had travelled the world and encountered the glorious variety of kinds of English (like Singapore's 'Singlish', whose structure is so Chinese). It helped prepare me for all that journeying, taught me cool words like 'relexification' -- happening all around me in Bangalore -- and freed me of some of the dafter prejudices of my schoolteachers. Not necessarily always right (don't get in the middle of a fight between linguistics profs!) but always stimulating.
African-American Vernacular English is the most distinctive subdialect of American English, with a peculiar phonology, morphology and syntax ("You is a racist", "he black"). Dillard thinks that it is derived from a West African English-based creole, which was brought to America by some slaves and was spoken on plantations; other linguists, such as John McWhorter, disagree. I didn't know that some slaves brought to America the Akan (a Ghanian-Côte d'Ivoirean people) tradition of naming children by the day of the week of their birth!
It's not sloppy or ignorant speech. Before the term "Ebonics" came along, Dillard labelled it "BEV", or "Black English Vernacular". It's a dialect, with its own grammatical rules. I wish everyone would read this...