Michael Haykin has suggested some books for those beginning to read on Patristics. Pastristics is Latin for “of the fathers”. It deals with some of the most important figures in history who have contributed to the rise of Christianity. An example would be Augustine.
I would start with Robert Louis Wilken, The Spirit of Early Christian Thought: Seeking the Face of God (New Haven: Yale University Press, 2003). Then: Christopher A. Hall, Reading Scripture with the Church Fathers (Downers Grove, Illinois: InterVarsity Press, 1998). Finally, a third book that is a gem, but not easy is Jaroslav Pelikan, The Christian Tradition. A History of the Development of Doctrine. Vol. 1: The Emergence of the Catholic Tradition (100-600) (Chicago/London: University of Chicago Press, 1971).
And do not forget getting into the Fathers directly. Start with Augustine, Confessions, trans. R.S. Pine-Coffin (Harmondsworth, Middlesex: Penguin, 1961) [this is the translation I like, but there are others]. Or read through an excellent collection by Steven A. McKinion, ed., Life and Practice in the Early Church. A Documentary Reader (New York/London: New York University Press, 2001). Another favourite of mine is Basil of Ceasarea, On the Holy Spirit, trans. David Anderson (Crestwood, New York: St. Vladimirs Press, 1980).
For a good overview of the period, see the relevant pages in Tim Dowley ed., Introduction to the History of Christianity (1990 Rev. ed.; repr. Minneapolis: Fortress Press, 1995) and for the key leaders, see John D. Woodbridge, ed., Great Leaders of the Christian Church (Chicago: Moody Press, 1988). The latter is regrettably out of print, but second-hand copies can be gotten easily. I have also had published Defence of the Truth: Contending for the truth yesterday and today (Darlington, Co. Durham: Evangelical Press, 2004), which deals with theological challenges faced by the Ancient Church.