Today in some parts of the world is the commemoration for St. Gregory of Nyssa, one of the great Cappadocian fathers, brother of St. Basil the Great, one of the most brilliant minds in the history of the Church (although very bad at practical matters, by everyone's admission including his own and his brother's). His work On Not Three Gods, a tight and closely reasoned summary of the Cappadocian view of the Trinity, has been very big influence on my own view of the Trinity; if there are any theological works on the Trinity that I wish people discussing the subject would closely read first, they would be Basil's book on the Holy Spirit, Augustine's on the Trinity, and Gregory's on Not Three Gods.
Another important Nyssan work is his Life of Macrina, a spiritual classic. St. Macrina was the sister of Basil and Gregory, and although she left behind no writings, her mark on the world, known through her brothers, is significant. It is another work deserving wide readership.
One of his most important works, The Life of Moses, does not appear to be online. In it Gregory uses the life of Moses as a template for understanding spiritual progress. First we begin in the darkness of ignorance; then we find spiritual illumination; and this spiritual illumination leads us to the darkness of contemplation.