First, you should note that Garry Wills is not actually the author of this book. Saint Augustine wrote his Confessions in Latin between 397 and 400 AD. Garry Wills is one of several writers who have translated this work into English; thus he is not the author, but the translator.
The work was intended as a spiritual autobiography showing how Augustine progressed from a pagan childhood to a Christian adulthood. Augustine was born to a moderately wealthy pagan father and a Christian mother, Monica. His account of his childhood, especially the episode of stealing the pears, is intended to show the universality of original sin. His baptism on his mother's insistence lay the groundwork for his later conversion.
As a young boy and teenager, he received a typical education in the seven liberal arts, including a foundation of grammar, dialectic, and what we now would term general humanistic studies. He disliked studying Greek grammar and was lazy in his Greek studies, resulting in frequent corporal punishment and giving him a lifelong distaste for perfecting his knowledge of the Greek language, despite his interest in certain Greek philosophers and theologians.
He pursued advanced studies in rhetoric at Carthage, developing the skills in speaking and writing that would serve him well as a preacher in the future. Despite his Christian mother, his adolescence and early adulthood were a period of spiritual questing in which he explored neoplatonism and Manichaeism. At this period he entered into a relationship with a woman and had an illegitimate child.
Monica's prayers and devotion led to a deathbed conversion of Augustine's father. Although Augustine remained a heretic, Monica persisted in keeping her son company, even when he traveled to Rome, and praying for him, and her gentle persistence was one of the causes of his eventual conversion back to Christianity.