Final answer:
St. Ambrose advised Augustine to read the prophet Isaiah as preparation for his baptism, fitting due to Isaiah's significant role in Christian theology.
Step-by-step explanation:
St. Ambrose recommended that Augustine prepare for baptism by reading the works of the prophet Isaiah. Augustine of Hippo, who was a significant figure in early Christianity, was influenced to convert to the faith after reading about the life of Antony of the Desert and the guidance from Ambrose, Bishop of Milan. Isaiah's prophecies are central in Christian theological study as they are often interpreted as foretelling the coming of Jesus Christ, making them a logical choice for catechetical reading in preparation for baptism in the early Christian church.