The following are sources that I highly recommend (watch for frequent additions):
Barclay, William. Turning to God: A Study of Conversion in the Book of Acts and Today. Grand Rapids, Mich.: Baker Book House, 1964.
Daniélou, Jean Cardinal. “The Sacraments and the History of Salvation.” Letter and Spirit 2 (2006): 203-216.
Dujarier, Michel. The Rites of Christian Initiation: Historical and Pastoral Reflections. New York: William H. Sadlier, Inc., 1979.
Finn, Thomas M. From Death to Rebirth: Ritual Conversion in Antiquity. New York: Paulist Press, 1997.
Grant, Robert M. “Development of the Christian Catechumenate.” In Made, Not Born: New Perspectives on Christian Initiation and the Catechumenate, from the Murphy Center for Liturgical Research, 32-49. Notre Dame, Ind.: University of Notre Dame Press, 1976.
Harmless, William. Augustine and the Catechumenate. Collegeville, Minn.: The Liturgical Press, 1995.
Hovda, Robert W. “Hope for the Future: A Summary.” In Made, Not Born: New Perspectives on Christian Initiation and the Catechumenate, from the Murphy Center for Liturgical Research, 152-167. Notre Dame, Ind.: University of Notre Dame Press, 1976.
Hughes, Kathleen. Saying Amen: A Mystagogy of Sacrament. Chicago: Liturgy Training Publications, 1999.
Johnson, Maxwell E. The Rites of Christian Initiation: Their Evolution and Interpretation. Collegeville, Minn.: The Liturgical Press, 1999.
Justin Martyr, “The First Apology of Justin.” In Ante-Nicene Fathers: The Writings of the Fathers Down to A.D. 325, Vol. 1, ed. Alexander Roberts and James Donaldson, 163-187. N.P.: Christian Literature Publishing Company, 1885–1900. Reprint, Peabody, Mass.: Hendrickson Publishers, 1999.
Kavanagh, Aidan. “Christian Initiation of Adults: The Rites.” In Made, Not Born: New Perspectives on Christian Initiation and the Catechumenate, from the Murphy Center for Liturgical Research, 118-137. Notre Dame, Ind.: University of Notre Dame Press, 1976.
Kavanagh, Aidan. “Unfinished and Unbegun Revisited: The Rite of Christian Initiation of Adults.” Worship 53 (July 1979) 327–40. Quoted in William Harmless. Augustine and the Catechumenate. Collegeville, Minn.: The Liturgical Press, 1995.
Keifer, Ralph A. “Christian Initiation: The State of the Question.” In Made, Not Born: New Perspectives on Christian Initiation and the Catechumenate, from the Murphy Center for Liturgical Research, 138-151. Notre Dame, Ind.: University of Notre Dame Press, 1976.
Kreider, Alan. The Change of Conversion and the Origin of Christendom. Christian Mission and Modern Culture, eds. Alan Neely, H. Wayne Pipkin, and Wilbert R. Shenk. Harrisburg, Penn.: Trinity Press International, 1999.
Mazza, Enrico. Mystagogy: A Theology of Liturgy in the Patristic Age. Translated by Matthew J. O’Connell. New York: Pueblo Publishing Company, 1989.
Milavec, Aaron. The Didache: Text, Translation, Analysis, and Commentary. Collegeville, Minn.: A Michael Glazier Book published by the Liturgical Press, 2003.
Mitchell, Nathan D. “Dissolution of the Rite of Christian Initiation.” In Made, Not Born: New Perspectives on Christian Initiation and the Catechumenate, from the Murphy Center for Liturgical Research, 50-82. Notre Dame, Ind.: University of Notre Dame Press, 1976.
Regan, David. Experience the Mystery: Pastoral Possibilities for Christian Mystagogy. Collegeville, Minn.: The Liturgical Press, 1994.
Satterlee, Craig Alan. Ambrose of Milan’s Method of Mystagogical Preaching. Collegeville, Minn.: The Liturgical Press, 2002.
Turner, Paul. The Hallelujah Highway: A History of the Catechumenate. Chicago: Archdiocese of Chicago, Liturgy Training Publications, 2000.
Webber, Robert E. Ancient-Future Evangelism: Making Your Church a Faith-Forming Community. Ancient-Future Faith Series. Grand Rapids, Mich.: Baker Books, 2003.
Webber, Robert E. Journey to Jesus: The Worship, Evangelism, and Nurture Mission of the Church. Nashville: Abingdon Press, 2001.
Yarnold, Edward. The Awe-Inspiring Rites of Initiation: The Origins of the R.C.I.A., 2nd ed. Collegeville, Minn.: The Liturgical Press, 1994.