Twelve Years a Slave
The book is true, but Northup and his publisher, Derby, were influenced by Uncle Tom's Cabin by Harriet Beecher Stowe, which was published one year earlier.  The book is dedicated to Harriet Beecher Stowe.  They marketed the book as "another Key to Uncle Tom's Cabin," meaning that, like Stowe's book, A Key to Uncle Tom's Cabin, it explained and supported Uncle Tom's Cabin, proving that Stowe had not lied about how bad slavery was.