Stowe published A Key to Uncle Tom's Cabin the year after Uncle Tom's Cabin appeared in book form. The non-fiction work collects documentary evidence to back-up Stowe's portrayal of American chattel slavery.
Source: Stowe, Harriet Beecher. A Key to Uncle Tom's Cabin. Boston: John P. Jewett, 1853. Archive.org. https://archive.org/details/akeytouncletoms01stowgoog