Ireneaus & Tertullian and the "Rule of Faith"

An Anglican acquaintance is exploring Catholicism and inquiring into what was meant by the "Rule of Faith" as taught by Ireneaus and Tertullian. He is attempting to determine specifically what relevance this might have in defining what is (or what should be considered) authoritative for Christians today. He is reading Jaroslav Pelikan's The Emergence of the Catholic Tradition (100 - 600), but is interested in learning what the best Catholic books on Ireneaus and Tertullian and their views of the "Rule of Faith" in the early Church Suggestions?