In reading holy books we receive many lights and divine calls. St. Jerome says that when we pray we speak to God; but when we read, God speaks to us. St. Ambrose says the same: "We address him when we pray; we hear him when we read." In prayer, God hears our petitions, but in reading we listen to his voice. We cannot, as I have already said, always have at hand a spiritual Father, nor can we hear the sermons of sacred orators, to direct and give us light to walk well in the way of God. Good books supply the place of sermons. St. Augustine writes that good books are, as it were, so many letters of love the Lord sends us; in them he warns us of our dangers, teaches us the way of salvation, animates us to suffer adversity, enlightens us, and inflames us with divine love. Whoever, then, desires to be saved and to acquire divine love, should often read these letters of paradise.
--St. Alphonsus Liguori, The True Spouse of Jesus Christ