Today's Saint (April 11): Leo the Great
Leo is best known for his treatise (the Tome) on the Incarnation of Christ, which was accepted at the Council of Chalcedon in 451.
This work did much to solidify the orthodox view on the divine and human natures of our LORD, against such heresies as had been expounded by Eutyches (who denied the two natures of Christ) and Nestorius (who taught that in Jesus there were two persons).
Leo's love of worshipping God motivated him to use his talents in crafting many collects and other pieces of liturgical worship still in use today. "In the midst of the congregation he opened his mouth: and the Lord filled him with the spirit of understanding." (The day's introit, page F 26, The People's Anglican Missal).
The accompanying picture is a sculpture in Saint Peter's Basilica in Rome, and shows Leo the Great facing Attila the Hun. An interesting account of their "meeting" is contained in the Catholic Encyclopedia article linked below.
Books Worth Considering
NICENE AND POST-NICENE FATHERS: Second Series, Volume XII Leo the Great, Gregory the Great
Select Sermons Of S. Leo The Great On The Incarnation: With His Twenty-Eighth Epistle, Called The Tome (1886)
Leo the Great (The Early Church Fathers)
Oxford Dictionary of the Christian Church
The Concise Oxford Dictionary of the Christian Church