You should always start with the Seven Story Mountain.
His descriptions of loneliness and darkness are as poignant as St. Augustine Confessions... There is this realizations that you are the barrier between God and you and then the complete shame
of what that kind of awareness means. If we start there with either one's writings respectively Merton's Seven Story Mountain, and St. Augustine's Confessions, you can literally see, an affecting illustration of what changed them both into saints.
You see the ordinary in them and their suffering as they search.. and it's as real as our everyday lives. God chooses the most unlikely people.
Merton said: "When a man is conceived, when a human is brought into being a concrete subsisting thing, ...the God's image is minted into the world"
Think about that for a second... and then contemplate everything you know about God and what Christ did.