Books on Mental Prayer specifically:
Into the Deep by Dan Burke (simple and short book on mental prayer)
Conversation With Christ: The Teaching of St. Teresa of Avila About Personal Prayer by Peter Thomas Rohrbach
Time for God by Fr. Jacques Philippe
Prayer Primer: Igniting a Fire Within by Fr. Thomas Dubay
Deep Conversion, Deep Prayer by Fr. Thomas Dubay
Fire Within: St. Teresa of Avila, St. John of the Cross, and the Gospel on Prayer by Fr. Thomas Dubay
A Catechism of Mental Prayer by Joseph Simler
The Ways of Mental Prayer by Rev. Dom. Vitalis Lehodey
Difficulties in Mental Prayer by M. Eugene Boylan, O.C.R.
Great Books for Prayer
The Ignatius Bible: Revised Standard Version - Second Catholic Edition
The Catechism of the Catholic Church
The Better Part by Fr. John Bartunek (4-part series great for mental prayer)
Life of Christ by Fulton J. Sheen
Story of a Soul by St. Therese of Lisieux
The Interior Castle by St. Teresa of Avila
The Way of Perfection by St. Teresa of Avila
The Imitation of Christ by Thomas a Kempis
The Secret of the Rosary by St. Louis de Montfort
True Devotion to the Blessed Virgin Mary by St. Louis de Montfort
Dark Night of the Soul by St. John of the Cross
Trustful Surrender to Divine Providence by Fr. Jean-Baptiste Saint-Jure and Blessed Claude de la Colombiere
Abandonment to Divine Providence by Jean-Pierre de Caussade
The Spiritual Combat and a Treatise on Peace of Soul by Lorenzo Scupoli
The Glories of Mary by St. Alphonsus Liguori
How to Converse With God by St. Alphonsus Liguori
The Dialogue of Saint Catherine of Siena by Saint Catherine of Siena
Into the Deep by Dan Burke (simple and short book on mental prayer)
Conversation With Christ: The Teaching of St. Teresa of Avila About Personal Prayer by Peter Thomas Rohrbach
Time for God by Fr. Jacques Philippe
Prayer Primer: Igniting a Fire Within by Fr. Thomas Dubay
Deep Conversion, Deep Prayer by Fr. Thomas Dubay
Fire Within: St. Teresa of Avila, St. John of the Cross, and the Gospel on Prayer by Fr. Thomas Dubay
A Catechism of Mental Prayer by Joseph Simler
The Ways of Mental Prayer by Rev. Dom. Vitalis Lehodey
Difficulties in Mental Prayer by M. Eugene Boylan, O.C.R.
Great Books for Prayer
The Ignatius Bible: Revised Standard Version - Second Catholic Edition
The Catechism of the Catholic Church
The Better Part by Fr. John Bartunek (4-part series great for mental prayer)
Life of Christ by Fulton J. Sheen
Story of a Soul by St. Therese of Lisieux
The Interior Castle by St. Teresa of Avila
The Way of Perfection by St. Teresa of Avila
The Imitation of Christ by Thomas a Kempis
The Secret of the Rosary by St. Louis de Montfort
True Devotion to the Blessed Virgin Mary by St. Louis de Montfort
Dark Night of the Soul by St. John of the Cross
Trustful Surrender to Divine Providence by Fr. Jean-Baptiste Saint-Jure and Blessed Claude de la Colombiere
Abandonment to Divine Providence by Jean-Pierre de Caussade
The Spiritual Combat and a Treatise on Peace of Soul by Lorenzo Scupoli
The Glories of Mary by St. Alphonsus Liguori
How to Converse With God by St. Alphonsus Liguori
The Dialogue of Saint Catherine of Siena by Saint Catherine of Siena