Confessions by St. AugustineR.S. Pine-Coffin, tr. (A classic work with a contemporary sense of the struggle with rising above the sensual. First step: the unembarrassed disclosure of sin.).
Theology for Beginners  by Frank Sheed (One of the best presentations of the central tenets of the Christian faith ever written.)
A Map of Life: A Simple Study of the Catholic Faith by Frank Sheed (Short work that helps think through a master plan of life by looking through the eyes of Catholic faith.)

Laying a Solid Foundation for Spiritual Formation

Life of Christ  by Fulton Sheen (A look at the life of Christ through the Gospels.)
Introduction to the Devout Life  by St. Francis de Sales, C.O., O.F.M., (A classic guidebook to the deeper life in God by one of the most influential figures on the founders of the 17th century French Catholic school of spirituality).
Frequent Confession: Its Place in the Spiritual Life by Benedict Bauer, O.S.B. (Frequent confession is an important means to the interior life. This book makes a strong argument for the necessity of frequent confession.) 
This Tremendous Lover  by Eugene Boylan, O.C.R. (An inspiring, affective look at the process of turning one's life over to God.)

Going a Little Deeper

The Fulfillment of All Desire: A Guidebook for the Journey to God Based on the Wisdom of the Saints  by Ralph Martin (A modern work that draws upon the teaching of seven Doctors of the Church to feed the deep desire to journey toward God. Introduces one to the rich patrimony of the Church for spiritual direction.)
Abandonment to Divine Providence  by Jean-Pierre de Caussade, S.J. (The beginning of deeper conversion: learning to embrace the sacrament of the present moment.)
The Mother of the Saviour: And Our Interior Life by Reginald Garrigou-Lagrange, O.P. (Perhaps the best work on the Blessed Virgin and her role in relation to us ever penned.)
Interior Castle by St. Teresa of Avila (St. Teresa's vision of a castle composed of seven chambers, or "mansions," each representing a different stage in the development of the soul. A classic work for those interested in a deeper life of prayer.)
The Way of Perfection by St. Teresa of Avila (Another classic by the Carmelite nun and spiritual mother of St. Alphonsus. Detailed directions on the achievement of spiritual perfection based on three essentials — fraternal love, detachment from material things, and true humility.)

For Serious Study and Reflection

Self-Knowledge and Self-Discipline by B.W. (Basil William) Maturin (A provocative and challenging introduction to the underlying necessity of self-knowledge and self-discipline for overcoming sin and advancing toward God.)
The Spiritual Life: A Treatise on Ascetical and Mystical Theology  by Adolphe Tanquerey, S.S. (Going deeper in the pursuit of Christian perfection. Undoubtedly the finest, most comprehensive and best respected one volume treatise (manual) on the spiritual life ever written, by an American master of the 17th century French school of spirituality. For all the early years of Catholic growth in America this book was used for training priests.)
The Three Ages of the Interior Life  (2 vols.) by Reginald Garrigou-Lagrange, O.P. (As contemporary as one needs to go for direction on the interior life. This work makes every book written after it look shallow.)
My Daily Bread: A Summary of the Spiritual Life   by Anthony Paone, S.J. (An excellent and simple summary and popularization of the stages of the spiritual life for daily use.)
Fire Within: St. Teresa of Avila, St. John of the Cross and the Gospel on Prayer by Thomas Dubay, S.M. (The fruit of many years of spiritual direction, Fr. Dubay here synthesizes the teachings of two of the greatest teachers and mystics in the Catholic tradition.)
Prayer Primer: Igniting the Fire Within  by Thomas Dubay, S.M. (A simple but powerful primer on developing the practice of a serious prayer life.)
The Better Part: A Christ-Centered Resource for Personal Prayer by John Bartunek, LC (This is simply an outstanding guide to daily prayer and the practice of Gospel meditation. Each meditation is about three pages long and looks at a specific Gospel passage. A great introduction to structured meditation, or Lectio Divina.)