The White Man's Burden. Winthrop Jordan. 1974.

In this book, Winthrop Jordan argues that slavery and racism were mutually reinforcing and that, as a result, they generated each other simultaneously. In other words, it is not possible to see slavery preceding racism or vice versa. Over the course of time, skin color played a factor as blackness became associated with dirt, filth, and, by extension, evil. Blackness was not, however, considered a prerequisite for slavery. It just so happens that West Africans became more susceptible to exploitation for various reasons other than race. Slavery was reinvigorated as a labor source with the cotton boom in the South. It was the profitability of cotton and other cash crops which helped to entrench slavery.