Final answer:
Cicero's Hortensius is the book that significantly influenced Augustine's intellectual journey, leading him toward his Christian conversion.
Step-by-step explanation:
The book that caught Augustine's attention and had a profound impact on his intellectual development was Cicero's Hortensius. This work, which is unfortunately lost except for a few fragments, inspired Augustine with its call to philosophy and the pursuit of truth, setting him on a path toward his eventual conversion to Christianity. Although the Bible, Plato's Republic, and the Manichean scriptures also played significant roles in his life, it was Cicero's exhortation that initially ignited his love for philosophy and his quest for a deeper understanding of the truth.