Searching For And Maintaining Peace: A Small Treatise On Peace Of Heart, by Father Jacques Philippe has proved a source of strength and consolation for many parishioners since Westminster Abbey began to promote it a few years ago. It's also inexpensive—about $8 if part of a $25 order shipped free. He has also written several other fine small books, though only one—Time for God—seems available on the internet at the moment.
Anything by the late Father Thomas Dubay is well worth reading, but his Deep Conversion: Deep Prayer is particularly wonderful and shorter than his masterwork, Fire Within, which explores the teaching of Sts. Teresa and John of the Cross in the light of the Gospel.
The Protestant writer John Eldredge paints a dramatic picture of the Christian journey in Waking the Dead: The Glory of a Heart Fully Alive. It's a wake-up call to those who are unaware of the perils of that journey, but offers hope and courage to the reader. Written for anyone, it makes frequent reference to modern culture so appeals particularly to younger readers, especially men.
One of the most helpful books I have read about prayer is Ralph Martin's Hungry for God: Practical Help in Personal Prayer, recently revised and reissued. It can kick start prayer in those who don't know where to begin, or encourage a return to prayer in those who need a boost. He has also written a much heavier book (in more than one sense) called The Fulfillment of All Desire. This instant classic lives up to its subtitle, A Guidebook for the Journey to God Based on the Wisdom of the Saints, offering an excellent overview of the spiritual masters we call the Doctors of the Church, in language anyone can follow despite its depth. Although a solid introductory work, The Fulfillment of All Desire turns the reader towards prayer, not just study.
Enter Father Benedict Groeschel's name in the search box on Amazon or Chapters, and you'll find a host of books that will deliver what their titles promise. Particularly helpful to those facing great challenges are the recent Tears of God: Persevering in the Face of Great Sorrow or Catastrophe and his Arise from Darkness: What to Do When Life Doesn't Make Sense.
Not to be forgotten are classics like The Imitation of Christ and the Confessions of St. Augustine, widely available in many different (though not equally good) translations. The Introduction to the Devout Life by St. Francis de Sales, especially in a good translation, is food for thought and prayer. St. Teresa of Avila, despite her lofty reputation, is not hard to read; the Way of Perfection is excellent spiritual reading.
"Do the right spiritual reading, and prayer will look after itself." Abbot Eugene Boylan