Lydia Maria Francis Child, "Slavery’s Pleasant Homes" (1843) from American Antislavery Writings: Colonial Beginnings to Emancipation:
Whenever I find myself discussing the culture and morals of dead people, I often hear the back-handed defense that so-and-so was no worse than the majority at that time. Yes, X said bad things about Indians, but he didn't really say anything out of the ordinary for that time; yes, Y was an anti-Semite, but he could've been much worse--he was really a middle-of-the-road anti-Semite for that time; etc.
While that might be true, it's always nice to find someone historical who is not only advanced for their time, but pretty advanced for our time too. So Lydia Maria Francis Child was an abolitionist; but she also supported interracial marriage and full citizenship for African Americans. Imagine that: at a time when many abolitionists start off their tracts by conceding that blacks aren't really ready now (or ever) for citizenship, Child in the 1820s takes a position more progressive than certain state governments of the 1960s.
The anti-slavery position finds its way into this story of Child's, which nicely lays out a bunch of terrible issues with slavery: the difficulty of slaves forming families; the weirdness of whites owning their black siblings; the effect of rape both on the slaves and on the white families who have to deal with that; the rivalries of slaves; and the untold stories of heroism and love that get elided by the newspapers that focus only on the white people.
It is, in other words, a microcosm of what Harriet Beecher Stowe will put in Uncle Tom's Cabin. (Well, it's a microcosm of many of the issues.) The main differences are that Child tells the story in summary, at a bit of a remove, whereas Stowe dramatizes her story (and adds more characters and plots); it's easy to imagine Child's story as an important example in an essay decrying slavery. Which in a sense it is: this story was published in an anti-slavery book, as one of the many examples of reasons to support abolition.