Are you referring to the book The Narrative of the Life of Frederick Douglass by Frederick Douglass? This work is by Douglass himself, a former slave turned abolitionist, writer, statesman, minister, reformer, etc. The work American Slavery as it is: Testimony of a Thousand Witnesses is a compilation of the abuses of slaves that were recorded by the slaveholders themselves! They used to keep track of how they brutalized their slaves. These records were published in southern newspapers in the hopes of sharing "helpful" information among slaveholders on how to deal with slaves.
The Reverend Theodore D. Weld and Angelina and Sarah Grimke from South Carolina disovered these reports and exposed them. When the slaveholders realized this, they stopped writing about them publicly. See the link below to read some of these reports. You can read some information about Weld, a well-known reformer, here on enotes, as well as the Grimke sisters, who actually were daughters of slaverholders!