On Cleaving to God...just recently read it and it has helped me tremendously with my interior life.
I listen to it online frequently. It's like an out of body experience with my soul uniting to God alone.
Trustful Surrender is a must read every. single. year. of my life or nothing makes any sense at all.
The Spiritual Exercises and the Imitation were life changing but it's been many years now...
Mere Christianity was part of my Jesuit College course and while there are problems with it, the diagram of the circles which replace Jesus as the center of one's life instead of self was very inspiring.