Alphonsus was a prolific writer. So many of his letters deal with the publishing of his works. Probably his best known works are on prayer, Mary, practicing the love of Jesus Christ, and Eucharistic adoration.
Prayer, love, his relationship with Christ, and his first-hand pastoral experience of everyday needs of the faithful made him one of the truly great masters of the interior life as well as a Doctor of the Church.
But it is his work Moral Theology which rings loudly with the loving pastoral care of his flock. Augustine's "all things in moderation" seems to be his mantra when dealing with and advising others on dealing with the frail faithful. He opposed the vanilla legalism prevalent in the theology around him (which he saw as the result of elitism) and rejected the strict "rigorism" that it produced. He also sought to avoid a laxness which is the usual opposite reaction to rigidity, exemplifying Jesus' point that love, not law or permissiveness, is the basis for morality.
It has given me much pleasure to know that you will confide the revision to a Jesuit Father, for were you to choose one of the Dominican Fathers, who at present follow Father Concina, he would censure as lax many opinions which I have advocated. You know that, as a general rule, I adhere to the teaching of the Jesuits (not of the Dominicans), and their opinions are neither lax nor rigorous, but rather the golden mean. And if I do maintain one or the other rigorous opinion against some Jesuit, I hold it nearly always on the authority of other Jesuits. From this Society, I confess, I have learned what little I have in my books; for they have always been (as I never cease to declare) and are yet the masters in Moral Theology.
-- From Letter 10