The Confessions of St. Augustine, which details how he came to faith. I was also struck by Augustine's quite graceful Latin style. (The translation by Henry Chadwick is good if you do not read Latin.)

Apologia Pro Vita Sua -- either "a defense of his life" or "an explanation of his life" -- by Cardinal Newman. In the 1860s, Charles Kingsley (best known for the novel The Water-Babies) attacked Newman for repeatedly saying one thing at one time, and another -- even the opposite -- at another time. Newman wrote about how he grew spiritually and intellectually, explaining how and why he came to change his mind on various subjects. Considerably later, and in quite different circumstances, G. K. Chesterton wrote, "A man may well be less convinced of a philosophy from four books, than from one book, one battle, one landscape, and one old friend." This echoes throughout Newman's Apologia.

Dietrich Bonhoeffer's The Cost of Discipleship. About what the Christian is called to do if his or her claim to being a Christian is genuine. Bonhoeffer himself was executed by the Nazis, basically because he took his Christianity seriously. The section on "cheap grace" is particularly noteworthy.

Several books by Thomas Merton, especially Conjectures of a Guilty Bystander, which taught me much about Christian mysticism; and Zen and the Birds of Appetite, which introduced me to Zen.

One not on spiritual growth is Papal Sin by Garry Wills. This is about honesty and the lack of it in the Vatican. It confirmed many of my own ideas -- basically that all too often, the papacy does not teach or preach honestly. I know Wills, and he and I see eye-to-eye on this subject. (Wills, interestingly enough, is quite conservative politically. But I forgive him his lapse in judgment.)