This list is by no means exhaustive and, in fact, is rather limited, but it should serve as an introduction to many of the well-known classics. Several notable works were excluded because they were no longer in print.
CONFESSIONS (c. 400) Augustine
Long considered the classic of all time, many modern readers will find this book difficult to read with scattered wisdom. The genre itself will seem unfamiliar and slightly wordy to many evangelicals. While every Christian should eventually read this book--it's a literary classic as much as a spiritual one--it's not the one we'd recommend you begin with, though it is the most easy to locate.
CITY OF GOD (c. 400) Augustine
Very lengthy, but with some incredibly rich passages. Augustine challenges neo-paganism by discussing religious lore, philosophy, theology, and history. This classic is widely recognized by all traditions, and, like Confessions, is one that most Christians should aspire to read at some point.
LADDER OF DIVINE ASCENT (c. 640) John Climacus
The classic of eastern Christendom, written for monks, this book calls for a high Christian commitment. It's an ascetic handbook, so at times the message may seem harsh, but, read in context, the book is definitely worthy of the attention it has received.
ON LOVING GOD (c. 1126-1141) Bernard of Clairvaux
Bernard of Clairvaux was a giant among his peers. In many ways, his spiritual journey was stereotypical: harsh asceticism and legalism in his youth, maturing to great love and intimacy with God in his later years. You can't go wrong with a classic that bases the spiritual life on loving God.
THE CLOUD OF UNKNOWING (late 14th c.) Author Unknown
This work is considered a classic of mystical spirituality, and evangelicals may find the full "program" completely unattainable, of little interest or benefit. However, there are many gems scattered throughout for those who take the time to read it. I once remarked that I read The Cloud more for the quotes than for the general concepts.
IMITATION OF CHRIST (c. 1418) Thomas a Kempis
This is probably one of the most popular spiritual classics of all time and for good reason. It's a treasure house of insights. A' Kempis focused on rigorous spiritual training as a necessary part of Christian living. His work is thus a good counter to "soft" Christianity, but others might say it goes too far in this regard.
THE SPIRITUAL EXERCISES OF ST. IGNATIUS LOYOLA (1548) Ignatius Loyola
Full of very practical advice for monks, this book also offers many helpful insights for evangelicals. With some modifications, evangelicals can benefit by systematically going through the exercises.
ASCENT OF MT. CARMEL (c. 1587) John of the Cross
A mystical classic. John was recognized as a highly gifted spiritual director (he was a follower of Teresa of Avila, but also served as her spiritual director during a three year period). In this work John provides many helpful insights, particularly into the dry stages of the Christian life. One of my favorite authors, John wrote with an unparalleled passion for God.
DARK NIGHT OF THE SOUL(late sixteenth century) John of the Cross
Another classic by John of the Cross. Though published under a different title than Dark Night of the Soul, the two volumes are really companions and should be read together for maximum benefit and understanding. Abridged versions of both works appear in th Paulist Press edition mentioned above.
THE INTERIOR CASTLE (1588) Teresa of Avila
This is a relatively short book on prayer, emphasizing spiritual visions leading to spiritual betrothal and marriage. A recognized classic on the devotional life and prayer.
INTRODUCTION TO THE DEVOUT LIFE (1609) Francis de Sales
A fantastic, very readable work. This was a unique spiritual book for its time, in that Francis wrote it for laypeople, not for those living in a religious community. His desire was to help the ordinary tradesman grow, recognizing that such a person needed different advice than that generally given to members of a religious community. This book is very practical with several helpful meditations, but some of these meditations will seem very "Roman Catholic" and evangelicals may choose to skip over these.
SIN AND TEMPTATION (1656-1667) John Owen
This is actually a compilation of three of John Owen's treatises, now brought together by Dr. James Houston. John Owen was a leading reformer, and the primary systematizer of Calvin's thought. His teaching on sin and temptation is must reading for every Christian, perhaps the very best I've ever read on this subject.
PENSEES (1670) Blaise Pascal
Pascal was a brilliant man, excelling in science, devotion, and his understanding of human nature. The Pensees is actually an unfinished collection of his thoughts, intended to be completed later as an exhaustive apology of the Christian life. It's current form is thus somewhat haphazard, but there is some incredibly rich material for those willing to wade through the collection.
PILGRIM'S PROGRESS (1678) John Bunyon
This has been an enormously popular work of fiction (allegory) from the day it was first published.
EXPERIENCING THE DEPTHS OF JESUS CHRIST (1685) Madame Guyon
Originally published as Short and Very Easy Method of Prayer, this book has impacted many influential Christians. It's one of the books I'd recommend people begin with if they haven't read the classics before. Jeanne Guyon focuses on abandonment to the Lord's will, and she knew what she was writing about--she lived a difficult, persecuted life. This book was actually banned by the Church of Rome in 1699, and then became very popular among the Protestants, including John Wesley.
THE PATIENCE OF THE PRESENCE OF GOD (1692) Brother Lawrence
Brother Lawrence was a very humble man with an extraordinary sense of living in God's presence. This little book is a treasure house of devotion and includes several letters and conversations regarding his wonderful awareness of God's presence. It's an excellent book with which to begin reading the classics.
CHRISTIAN PERFECTION (1704-1717) Fenelon
Fenelon is definitely one of my favorites. He wrote as a wealthy mystic living in the upper strata of French society. The temptations and spiritual struggles faced by the early eighteenth century French elite were remarkably similar to those faced by middle-class evangelicals today. This is one of the most helpful spiritual classics I've read. It's one you'll likely want to read over and over.
A CALL TO A DEVOUT AND HOLY LIFE (1728) William Law
This is a rigorous treatise written by a devout Puritan (he emphasizes grace a bit more in a later work entitled The Spirit of Love). It is very helpful, challenging and practical, and also one of my favorites, but it could be somewhat dangerous for a person who isn't rooted in grace and who may be prone to legalism.
A TREATISE CONCERNING RELIGIOUS AFFECTIONS (1746) Jonathan Edwards
Discusses those who are "truly pious" by examining and discussing various religious affections.
Ordering the Classics
I have found very few Christian bookstores that actually stock these works. However, armed with the information contained in this list, any bookstore (including a "secular" one) should be able to order the classics of your choice. The National Cathedral in Washington, D.C. has a fairly extensive collection for a bookstore its size.
Around the country, if your favorite Christian bookstore doesn't carry a desired title, look for: Roman Catholic bookstores, which are more likely to carry many of these titles; University bookstores with Catholic affiliations (Georgetown University in Washington, D.C., for instance, carries many titles); monasteries, which sometimes run small bookstores that carry many of the classics; Christian Book Distributors, a discount buying club, also carries many of these titles (particularly the Paulist Press editions) at reduced prices.
If you don't wish to purchase the book, inter-library loans are helpful. I've found few public libraries that actually carry more than one or two of these titles, but through inter-library loan, you can obtain most of them.