The following are the best overviews/introductions to Christian theological discussions of areas in social and political ethics:
Bauckham, R. (1989). The Bible in politics : how to read the Bible politically. SPCK.
A brilliant short study offering political readings of various biblical passages
Charles, R. (1998). Christian social witness and teaching : the Catholic tradition from Genesis to Centesimus Annus. Gracewing.
A two-volume guide to historical (vol 1) and modern (vol 2) Catholic social teaching by one of the leading experts in this field. The final chapter of volume 2 gives a very helpful guide to structure of Catholic thought.
Hay, D., A. (1989). Economics today : a Christian critique. IVP.
A useful introduction to Christian approaches to economic issues by an evangelical economist
Koyzis, D. T. (2003). Political visions and illusions : a survey and Christian critique of contemporary ideologies. IVP.
A helpful study of different political ideologies from an evangelical Reformed world-view perspective.
Maritain, J. (1954). Man & the state. Hollis & Carter.
Classic modern RC theological discussion of the role of the state
Niebuhr, R. (2001). Moral man and immoral society : a study in ethics and politics. [Full text on-line] Westminster John Knox Press.
Classic text by 'Christian realist' US theologian, originally published in 1932.
O'Donovan, J. L. and O. O'Donovan (1999). From Irenaeus to Grotius : a sourcebook in Christian political thought 100-1625. Eerdmans.
A brilliant resource providing primary texts in classical Christian political thought from Patristic to Early Modern along with introductory comments by editors.
O'Donovan, O. (1996). The desire of the nations : rediscovering the roots of political theology. Cambridge University Press.
A major study in political theology which studies Scripture in some detail and provides an historical account and defence of Christendom; heavy-going but stimulating and provocative throughout.
Scott, P. and W. T. Cavanaugh (2003). The Blackwell companion to political theology. Blackwell.
Wide-ranging collection of articles on biblical, historical, theological and contemporary themes relating to political theology
Stackhouse, M. L. (1995). On moral business : classical and contemporary resources for ethics in economic life. Eerdmans.
A similar volume to Lammers & Verhey but devoted to economic and business ethics
Wogaman, J. P. (2000). Christian perspectives on politics. Westminster John Knox Press.
Revised edition of text studying different Christian approaches to social and political issues
Yoder, J. H. (1994). The politics of Jesus : vicit Agnus noster. Eerdmans.
Originally published in 1972, a modern classic from this influential Anabaptist theologian. Others works of Yoder online here
See also the CMR historical and contemporary reading lists
Roman Catholic Social Teaching
The key papal encyclicals developing Roman Catholic Social thought are all available (and the more recent of them searchable) online:
Rerum Novarum (1891)
Quadragesimo Anno (1931)
Mater et Magistra (1961)
Pacem in Terris (1963)
Populorum Progressio (1967)
Octogesima Adveniens (1971)
Laborem Exercens (1981)
Sollicitudo rei socialis (1987)
Centesimus Annus (1991)