According to The Key to Uncle Tom's Cabin, Stowe talked with Clarke about his experiences as a slave before and during the composition of her novel. Although she spells his name wrong there, she also cites him as one of the sources for George Harris' character. Lewis Clarke's Narrative was first published by itself, in 1845. It was reprinted, as far as I can tell without revision, in the volume used here.
Narratives of the Sufferings of Lewis and Milton Clarke, Sons of a Soldier of the Revolution, During a Captivity of More Than Twenty Years Among the Slaveholders of Kentucky, One of the So Called Christian States of North America. Dictated by Themselves. (Boston: Published by Bela Marsh, 1846)