Complex sentence: Testimony of a Thousand Witnesses, an 1839 volume co-authored by abolitionists Theodore Dwight Weld and the Grimké sisters, which was a source for some of the content in "Uncle Tom's Cabin".
Split into simpler sentences: Testimony of a Thousand Witnesses, an 1839 volume co-authored by abolitionists Theodore Dwight Weld and the Grimké sisters. This was a source for some of the content in "Uncle Tom's Cabin".