For a brilliant and influential account of the contract theory, see Amy Dru Stanley, From Bondage to Contract.

In practice, however, the equivalence between contracts and freedom was never so clear.

Amy Dru Stanley, From Bondage to Contract: Wage Labor, Marriage, and the Market in the Age of Slave Emancipation (New York: Cambridge Univ. Press, 1998).

Kelly Sisson, "Bound for California: Chilean Contract Laborers and Patrones in the California Gold Rush, 1848-1852," Southern California Quarterly 90 (Fall 2008): 259-302.