Warning - this is NOT your conventional history novel. Whitehead adapts to historical events to fit his master plan. What's that plan you ask? I'd say the best way to discover it is by reading the book. Every time Whitehead steps outside the lines of history is where he makes his most salient points about slavery, and its milieu in approximately 1820's United States. This meta layer added extra meaning in how they affected the main character, Cora (a slave on a Georgia plantation) and those around her. 
 For those who want to know that that extra layer is... (view spoiler)[ Whitehead has crafted a contemporary (to 1820's America) equivalent of Gulliver's Travels, AKA Travels into Several Remote Nations of the World (he even references the work within the novel). Cora goes on a series of travels of her own, with the salient points being what she perceives about the lands she visits and how the inhabitants perceive her. To this end he appropriates some anachronistic events from later in African American history and uses them to highlight the similarities and differences in the societies Cora visits. (hide spoiler)] 
 I think it would do a great disservice to the book if taken purely on historical grounds as that would be beside the point. The essence of the story, to me, is a travelogue and I think Whitehead executes it brilliantly.