Today’s his feast, of course. One of my favorite saints for many reasons, including the fact of his determined resistance to God’s call, the expression of that call through the voice of the people of Milan, and then his full-spirited embrace of his duty when he discerned that yes, this was the way God wanted him to go.
A few St. Ambrose-related posts coming your way.
(Our 2011 trip to Milan. Ambrose on Repentance.)
First, the basics:

Artwork: St. Ambrose rebukes Theodosius by Daniel Mitsui. More here.
In depicting this event, I wanted to give emphasis to the ideas of repentance and forgiveness; in a quatrefoil above the door of the church, I drew King David confronted by the Prophet Nathan (whose parable appears in a smaller quatrefoil). The postures, clothing, and positions of these figures relate them to those of St. Ambrose and Theodosius below. The beginning of the psalm expressing King David’s repentance I wrote on the lintel. On the door is a sactuary knocker (in the form of a green man, similar to that knocker on Durham Cathedral) which suggests again the idea of a church as a place for the repentant. Both St. Ambrose and the Emperor wear purple vestments; the color is a symbol both of repentance and of imperiality.
The crosier carried by St. Ambrose is my own fanciful design; it has a carved-ivory head in the shape of a seahorse with snails for crockets, and terminates in a narwhal tusk.
A narwhal tusk!
We might begin with his hymn, Veni Redemptor gentium:

A Clerk of Oxford has a wonderful, thorough post on the hymn. Go here for that!
He’s in the Loyola Kids Book of Saints, under “Saints are People Who Change Their Lives for God.”

You can peek at the chapter here, at Google Books.
B16 at a General Audience, speaking about St. Ambrose:
Dear brothers and sisters, I would like further to propose to you a sort of “patristic icon”, which, interpreted in the light of what we have said, effectively represents “the heart” of Ambrosian doctrine. In the sixth book of the Confessions, Augustine tells of his meeting with Ambrose, an encounter that was indisputably of great importance in the history of the Church. He writes in his text that whenever he went to see the Bishop of Milan, he would regularly find him taken up with catervae of people full of problems for whose needs he did his utmost. There was always a long queue waiting to talk to Ambrose, seeking in him consolation and hope. When Ambrose was not with them, with the people (and this happened for the space of the briefest of moments), he was either restoring his body with the necessary food or nourishing his spirit with reading. Here Augustine marvels because Ambrose read the Scriptures with his mouth shut, only with his eyes (cf. Confessions, 6, 3). Indeed, in the early Christian centuries reading was conceived of strictly for proclamation, and reading aloud also facilitated the reader’s understanding. That Ambrose could scan the pages with his eyes alone suggested to the admiring Augustine a rare ability for reading and familiarity with the Scriptures. Well, in that “reading under one’s breath”, where the heart is committed to achieving knowledge of the Word of God – this is the “icon” to which we are referring -, one can glimpse the method of Ambrosian catechesis; it is Scripture itself, intimately assimilated, which suggests the content to proclaim that will lead to the conversion of hearts.
Thus, with regard to the magisterium of Ambrose and of Augustine, catechesis is inseparable from witness of life. What I wrote on the theologian in the Introduction to Christianity might also be useful to the catechist. An educator in the faith cannot risk appearing like a sort of clown who recites a part “by profession”. Rather – to use an image dear to Origen, a writer who was particularly appreciated by Ambrose -, he must be like the beloved disciple who rested his head against his Master’s heart and there learned the way to think, speak and act. The true disciple is ultimately the one whose proclamation of the Gospel is the most credible and effective.
Like the Apostle John, Bishop Ambrose – who never tired of saying: “Omnia Christus est nobis! To us Christ is all!” – continues to be a genuine witness of the Lord. Let us thus conclude our Catechesis with his same words, full of love for Jesus: “Omnia Christus est nobis! If you have a wound to heal, he is the doctor; if you are parched by fever, he is the spring; if you are oppressed by injustice, he is justice; if you are in need of help, he is strength; if you fear death, he is life; if you desire Heaven, he is the way; if you are in the darkness, he is light…. Taste and see how good is the Lord: blessed is the man who hopes in him!” (De Virginitate, 16, 99). Let us also hope in Christ. We shall thus be blessed and shall live in peace.
OSV, along with many other publishers at the time, collected these talks on the early Church Fathers into a volume. I wrote a study guide to go along with it, which is now available for a free download. So there you go – an easy personal or group study, at no cost to anyone for materials, since the talks are online at the Vatican website. Below are the pages for the Ambrose-Augustine chapter.