In the second volume of his book The Love of God and the Cross of Jesus (trans. Sister Jeanne Marie, O. P.; St. Louis, MO: B. Herder Book Co., 1951), ch. xii. "The Place of the Unitive Life and the Mystical Order," Rev. Fr. Reginald Garrigou-Lagrange, O.P., discusses the excellence of the writings of St. Alphonsus Liguori:

The spiritual works of St. Alphonsus are to be recommended because of their doctrinal certainty, the saintly unction found on every page and, above all, because of their eminently practical character. They are calculated to round out the great speculative views to which St. Thomas accustoms us, and to show how these ought to give direction to our daily actions in advancing toward perfection.

St. Alphonsus has been declared a doctor of the Church precisely because of his sure doctrine on moral and spiritual matters. He was given by God to His Church at a time when the Jansenist heresy was turning souls away from the contemplation of infinite mercy, when quietist errors tended to drive generosity out of the service of God, when the doctrines of Voltaire, Rousseau, and the Encyclopedists were ravaging the minds of men. In the sorry eighteenth century this great saint served as the voice of God reminding men of His indefeasible rights and even more of His boundless mercy, restating the greatness of the divine law and the Christian conscience. He gave new life to moral doctrine, then so often smothered under the vain discussions of casuists more interested in the material circuмstances of human acts than in the divine exigencies of the last end; and he re-established, for the greater good of souls, the confessional as the center of parochial life.

As to spirituality properly so called, he continued the work of the sanctification of the common people undertaken by St. Dominic and St. Francis of Assisi in the thirteenth century and continued by St. Francis de Sales in the seventeenth. The latter had, as it were, brought spirituality out of the cloister and among seculars but his Introduction to the Devout Life addressed only people of culture. St. Alphonsus Liguori, like the great popular preachers of the Middle Ages, brought himself down to poor folk, to country people, who were receiving all too little care, that he might teach them, in language wonderfully suited to their understanding, the beauty of the mysteries of the faith and of the Christian life.

And so here are the Englished works of the Saint I have been able to find on the internet (given in no particular order):

The Dignities and Duties of the Priest

The Great Means of Salvation and of Perfection

Miscellany

The Holy Eucharist

The Holy Mass

The Divine Office

Preaching

The Way of Salvation and of Perfection

The Incarnation, Birth and Infancy of Jesus Christ

Victories of the Martyrs

Abridged Sermons for All Sundays of the Year

The Passion and Death of Jesus Christ

The History of Heresies and Their Refutation, vol. 1

The History of Heresies and Their Refutation, vol. 2

The True Spouse of Jesus Christ, Vol. 1

The Glories of Mary[/font]

During my grandmother's last two years on earth, she reaped enormous spiritual fruit from daily listening to the book "Preparation for Death" of this great saint. She received his instructions with such child-like, innocent docility it was beautiful. If only we could all have such simple souls!  

Preparation for Death

More of his works can be found here:

Saints Books

His writing has led me to, and been my faithful companion, throughout my exploration of tradition. If only we could apply faithfully even a tenth of what he writes, we could all be great saints!