The chief editors of this encyclopedia, Dana and Ripley, were antislavery. One of their main contributors and editors working under them, Michael Heilprin, was a brave and outspoken Jewish opponent of slavery. The publishing company was named after David Appleton. He could not have owned slaves because he lived in Massachusetts and New York City after they had abolished slavery. His business was retail, including books, and so could not have included slaves. In any case, he retired before this encyclopedia was published.
More about Dana and Ripley: Before the Civil War, Dana and Ripley worked for Horace Greeley's newspaper the New York Tribune. The Tribune was an antislavery newspaper. Greeley supported Abraham Lincoln and helped get him elected. During the Civil War, Dana worked for the US War Department against the Confederacy. Dana was of big assistance to General Ulysses S. Grant.
I don't know where this rumor started about "D.Appleton owning slaves." That's complete nonsense.