You are reffering to Barracoon: The Story of the Last "Black Cargo"
It is a non-fiction book recounting the interview the author had with Cudjo Lewis, who was the last living survivor of the Middle Passage. This was the stage of the trangular trade in which millions of Africans were shipped to the new world s part of the Atlantic slave trade. It wasn't initially published because it was written in vernacular and because it detailed the involvment of other Africans in the slave trade, condemning their own.
The book was finally published in 2018.