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).
Gunther Peck, Reinventing Free Labor: Padrones and Immigrant Workers in the North American West, 1880-1930 (New York: Cambridge Univ. Press, 2000), 65.
Peck, Reinventing Free Labor, 51-53
Yuji Ichioka, "Japanese Immigrant Labor Contractors and the Northern Pacific and the Great Northern Railroad Companies, 1898-1907," Labor History (1980): 326, 330-31, 333, 336-37
Craig Storti, The Incident at Bitter Creek: The Story of the Rock Springs Chinese Massacre (Ames: Iowa State Univ. Press, 1991), 59-66.
Peck, Reinventing Free Labor, is a superb account of contract labor.