Pseudo-Dionysius is the name given to an unknown Christian theologian and philosopher, probably a Syrian monk, who wrote around the year 500. He presented his works as those of Dionysius the Areopagite, Paul's Athenian convert, and they carried apostolic weight for a thousand years. His Mystical Theology founded the apophatic, or negative, way of speaking about God; our library also holds On the Divine Names and The Celestial Hierarchy.
Free: get it — here or in the Christian Classics app — and it's yours to read and listen to on every device you sign in to. About a tenth of it can be read here first, free.
The Celestial Hierarchy, attributed to Dionysius the Areopagite and written around AD 500, describes the nine orders of angels and their ministry of passing God's light downward through the ranks of heaven and the church. It founded the medieval science of angels and much of the church's language about hierarchy, symbol, and worship. A short, dense, influential treatise.