Both of these books, especially Uncle Tom's Cabin, helped make slavery into a moral issue for many Northerners. Many, perhaps most, Northerners had regarded slavery as the "peculiar institution," something that was unique to the South, and perhaps best managed by them. These books, however, and the abolitionist movement in general, tried to bring the issue to life. They portrayed slavery as brutal, degrading, and destructive of the morals of all involved, including whites.
Douglass's Narrative is a thoughtful and insightful critique of this aspect of slavery. He takes aim at hypocritical Christians who own slaves, demonstrates how the institution turns even well-meaning people like Sophia Auld into would-be tyrants, and invited his readers to "see how a slave was made a man" in his climactic brawl with Mr. Covey. The book earned Douglass rapid notoriety, and provided a boost to the abolitionist movement that was already starting to elbow its way into mainstream politics.
The appeal of Uncle Tom's Cabin was not so much authenticity, as Harriet Beecher Stowe had little direct experience with slavery. Nevertheless, Stowe was also able to depict the horrors of slavery, including brutal masters, the sorrow of being sold southward, and the bravery required to make an escape. The book was undoubtedly maudlin and sentimental in its depiction of slavery, and may strike modern readers as condescending and stereotypical in its treatment of African-Americans, but this appealed to her readers. Uncle Tom's Cabin sold more copies than any book other than the Bible during its time. It, like Douglass's work, brought slavery home to a largely apathetic readership, and aroused significant antislavery sentiment.