There's a carefully researched article on the UK Guardian that deals with Northup's life, his book, other slave narratives and the times in which he was enslaved.
When slave narratives were rediscovered in the 20th century, the fact that most had been ghosted or edited by white people once again raised the question of their authenticity: many historians repeated the century-old charge that the narratives were exaggerated or fabricated by abolitionists. Unfortunately, much of the US coverage of McQueen's film has rehearsed these invidious questions, but the underlying truths of the atrocities of slavery are beyond dispute, and not altered by the fact that any narrative is, by definition, constructed.
In the case of Northup, his account was verified by the historian who recovered his story, a woman named Sue Eakin. Twelve years old when she discovered a copy of Northup's narrative in a local plantation in 1930, Eakin was intrigued to find it described the area in which she lived. Six years later, as a student at Louisiana State University, she found a copy of the book in a local bookstore. The owner sold it to her for 25 cents, telling her it was worthless: "There ain't nothing to that old book. Pure fiction." Eakin would devote her life, she later said, to proving him wrong.
Eakin set about discovering everything she could about Northup's life, tracking down its details, using the legal and financial records of the men who owned him to corroborate his account of his enslavement. (Northup himself quotes more than once from such records: "The deed of myself from Freeman to Ford, as I ascertained from the public records in New-Orleans on my return, was dated June 23d 1841.")
Unlike many slave narratives, Northup's named names: the people who mistreated him were still alive, and their own records substantiate the facts of his story. Eakin died in 2009; three years later amateur historian David Fiske published Solomon Northup: His Life Before and After Slavery. Between them, Eakin and Fiske established that Northup played a significant role in his book's composition, working closely with Wilson over the three months they wrote it. Fiske even found reports of corroboration made by Edwin Epps himself, from union soldiers who met him in Louisiana during the civil war: "Old Mr Epps yet lives, and told us that a greater part of the book was truth," they reported in 1866.