Joseph N. Moody. Church and Society (1953) 49. "The pontificate of Leo XIII marked the beginning of an official effort to restate the traditional social teaching of the Catholic Church in the context of modern industrialism. The social encyclicals did not initiate the movement. In large part they were the consequence of pioneering by social-minded lay and clerical Catholics in many of the countries affected by the new conditions. But with Leo's reign, the movement 'captured' the papacy in the way that the eleventh century reform had 'captured' Leo IX or the reform of the sixteenth century had Paul III. In 1878, the See of Peter was occupied by a man who was familiar with the broad social currents of the age and who was personally sympathetic to the needs of the industrial proleteriat. From this time on, social issues were to receive major papal attention."
Kathryn Lamboley. National Catholic Reporter (1979) 37. "When Leo became pope in 1878, there was little harmony between the people and the papacy. During the funeral procession of his predecessor, Pius IX, a cursing, stone-throwing mob followed the funeral cortege to the body's final resting place at San Lorenzo Outside the Walls and threatened to throw the dead pope's body into the Tiber River.... When (Leo) died on July 20, 1903, flags flew at half mast in major cities around the world. His proposals had gone to the heart of major human concern--justice for the worker."
Fogarty, Gerald P., S.J. "Leo XIII." Judith A. Dwyer, ed., The New Dictionary of Catholic Social Thought, Collegeville, MN: Liturgical Press, 1994, pp. 546-48.
Gargan, Edward T., ed. Leo XIII and the Modern World. New York: Sheed and Ward, 1961.
Gilson, Etienne. "Introduction." The Church Speaks to the Modern World: The Social Teachings of Leo XIII. Garden City, NY: Image Books, 1954, pp. 1-28.
Keane, J. J. "The Catholic Church and Economics." Quarterly Journal of Economics 6 (October 1891) 25-46.
Murray, John Courtney. "Leo XIII on Church and State: The General Structure of the Controversy." Theological Studies 14 (March 1953) 1-30.
Murray, John Courtney. "Leo XIII: Separation of Church and State." Theological Studies 14 (June 1953) 145-214.
Murray, John Courtney. "Leo XIII: Two Concepts of Government." Theological Studies 14 (December 1953) 551-67.
Murray, John Courtney. "Leo XIII: Two Concepts of Government II, Government and the Order of Culture." Theological Studies 15 (March 1954) 1-33.
Wallace, Lillian Parker. Leo XIII and the Rise of Socialism. Chapel Hill, NC: Duke University Press, 1961.