How do the economic texts on slavery differ from the narratives on slavery, and why is it important to analyse this?

In the introduction to his book Did Slavery Pay? (1971: xii), a collection of readings on the economic effects of slavery, Hugh G. J. Aitken discusses what we can learn from these texts. He says they have much to tell us about slavery, and about the plantation economy, and the South, but they have little to tell us about the black man (1971: xii). To get a fuller understanding of the subject the narratives on slavery are extremely useful. They paint a vivid picture of what life was like for black men, women and children at the time. However it is important to keep in mind the differences between and the limitations of both these kinds of sources….(short extract)