(Warning: Beware the used or library textbook with highlighting. Do the human race a favor and don't highlight your books.)
C. FitzSimons Allison, The Cruelty of Heresy: An Affirmation of Christian Orthodoxy, Morehouse, 1994.
Augustine, Confessions (selections). Available on-line.
Bart Ehrman, Lost Christianities, Oxford, 2003.
Stuart G. Hall, Doctrine and Practice in the Early Church, Eerdmans, 1991.
Cyril C. Richardson, ed., Early Christian Fathers, Macmillan, 1970.
Robert L. Wilken, The Spirit of Early Christian Thought, Yale, 2003.
F.L. Cross, The Oxford Dictionary of the Christian Church, 2nd ed. or 3rd ed., Oxford, 1983 or 1997 ; or Angelo Di Berardino, Encyclopedia of the Early Church, 2 vols., Oxford, 1992 .
Richard A. Muller, Dictionary of Latin and Greek Theological Terms, Baker, 1985.
Christopher A. Hall, Learning Theology with the Church Fathers, IVP, 2002.
Keith Hopkins, A World Full of Gods: The Strange Triumph of Christianity, Plume, 2001.
J.N.D. Kelly, Early Christian Doctrines, 3rd ed., HarperCollins, 1978.
T.F. Torrance, The Trinitarian Faith, T&T Clark, 1993.
William C. Placher, A History of Christian Theology, Westminster, 1983.
Peter Brown, Augustine of Hippo: A Biography, rev. ed., University of California, 2000 ; or Henry Chadwick, Augustine: A Very Short Introduction, Oxford, 2001.
For students improving their writing skills, my classes feature conditionally required reading.