Frederick Douglass's intended audience was white people, mainly in the north, as he wanted to convince them of the damaging effects of slavery and to convince them that slavery should be abolished. The two prefatory letters in the book, one written by Wendell Phillips and one written by William Lloyd Garrison, were intended to make sure the white readership of the book knew that Frederick Douglass was trustworthy. William Lloyd Garrison and Wendell Phillips were white abolitionists who had a lot of credibility among white audiences in the north, and their blessing to Douglass went a long way in making sure white northern readers took Douglass seriously.
Douglass's autobiography was intended to let his white audience know about the damage that slavery not only inflicted on slaves but the damage it also inflicted on white slave owners. For example, in telling the story of his slave owner, Sophia Auld, Douglass illustrated how slavery degraded a woman who was formerly kind to slaves (as she had never held slaves before). The white audience that read this account would, Douglass hoped, determine that slavery was contrary to their religious values and that slavery made the white people associated with it amoral. These arguments would, Douglass hoped, convince people to push for the abolition of slavery.