More background on the book is available in this article, including why it was only now published:
At best one could hope for something that was left by mistake or that unexpectedly survived the fire sufficiently intact.
> Estimates of the number of slaves today range from around 21 million to 46 million, depending on the method used to form the estimate and the definition of slavery being used.
> According to research provided by Emory University as well as Henry Louis Gates Jr., an estimated 12.5 million slaves were transported from Africa to colonies in North and South America.
Slavery was never abolished in the US, only restricted.