Many whites in the North supported slavery because they didn't understand the reality of what it was like for the enslaved. Many whites believed the ideology that the slaves were childlike creatures who were happy and well cared for by benign owners. Many whites, going along living their daily lives, didn't stop to imagine what it would be like to be owned by another person and subject to all their whims, with cruel punishments for disobedience. Many people didn't stop to note that sometimes the owner was a child, without a fully developed conscience, who was given a slave as a "gift."
Therefore, books that publicized and detailed the lives of slaves were important to changing hearts and minds. Former slave Frederick Douglass, for example, was at pains to counter arguments often used to justify slavery. For example, the seemingly happy singing of the slaves was often used to argue they were content with their lot: Douglass argued that the songs emerged from deep anguish over being held captive. He also dispelled the notion that slaves who said their masters treated them well were speaking the truth, telling the story of a slave who mistakenly complained about his master, only to be sold south to the cotton plantations.
Other whites, like Harriet Beecher Stowe, believed slavery would gradually wither and disappear and so did not worry too much about opposing it. It was only when politics seemed to be moving in the direction of strengthening slavery that Stowe rebelled and wrote Uncle Tom's Cabin. This book created a firestorm in favor of freeing the slaves immediately, showing that many people's support of slavery came from a lack of imagining what it was really like, rather than active belief in the institution.