What does Augustine struggle over throughout this book? How does that struggle end?

This is a little long but it needs to be in order to properly answer your question,

Book Eight of The Testimony tells the second most famous religious conversion story in Western literature, second only to that of Saint Paul, on which it is modelled...Augustine's internal conversion now must be matched by an external conversion. Augustine's way of thinking has changed, but he is still a man of the world, pursuing a high-powered career, planning to marry an heiress, and maintaining a lover on the side. Augustine cannot be satisfied with anything less than a total commitment to his new faith: withdrawal from the world, his career, his honors, and most painfully, from all sexual activity. In a sense, this commitment substitutes one kind of ambition for another: Formerly driven to excel in the world, Augustine is now driven to excel in his faith. Characteristically, Augustine cannot do anything by halves, and his painful deliberations over this radical change in his life are about to reach a crisis point. His return home to God is imminent, and he quotes the story of the Prodigal Son twice in 8.3, referring to the one who was dead, but is now alive.Augustine's final conversion at the end of Book 8 is the most famous episode from the Confessions. In a moment of intense emotional crisis, Augustine hears a mysterious child's voice chanting, "Take and read, take and read." When he does so, he encounters Romans 13:13–14, and the passage abruptly lays to rest all his doubts and fears about leaving his old life behind.