The film 12 Years a Slave was not based on a book written by a black man
The book is ghostwritten by a white associate of Solomon Northup who spruced it up. It has Northup making an absurd statement about negroes. The movie then exaggerates the book. 750k Imdb votes? Must have grossed $100m!
Simon Webb https://www.youtube.com/watch?v=zjNVMe8X2pQ
https://en.wikipedia.org/wiki/12_Years_a_Slave_(film)#Historical_accuracy