In this letter to educator and congressman Horace Mann (1796–1859), written 150 years ago this month, author Harriet Beecher Stowe (1811–1896) announces that she has completed what would become arguably the most influential novel in American history—Uncle Tom’s Cabin.

The work grew out of Stowe’s exposure to abolitionism, which had over time fostered her determined stance against slavery. She began composing the novel in 1850, as the national debate over slavery became ever more intense and angry. The storyline followed its protagonist, Uncle Tom, as one master sold him to another, until the abuses of his final owner, the villianous Simon Legree, brought about Tom’s death.

Uncle Tom’s Cabin first appeared in the magazine National Era in a series of installments between June 1851 and April 1852, where it enjoyed a relatively modest readership. When publisher J. P. Jewett issued the first printing of the book in March 1852, however, the sales soared. Ten thousand copies sold within the first week; in the first year, more than 300,000 copies sold in the United States alone.

Stowe’s subject matter had, of course, touched a raw nerve among American readers, and not all of her buyers were happy. The book enraged pro-slavery advocates, many of whom challenged her portrayal of the treatment of slaves on Southern plantations. Horace Mann, then serving in the U.S. House of Representatives, helped Stowe formulate her response to these charges. They gathered evidence of the atrocities of slavery, which resulted in a second book, A Key to Uncle Tom’s Cabin, published in 1853.

The Massachusetts Historical Society holds several of Harriet Beecher Stowe’s letters, including letters she wrote to William Prescott, Richard Henry Dana, and John A. Andrew. The MHS also holds a large collection of Horace Mann’s personal and family papers.