Pseudo-Dionysius the Areopagite 
Heavenly Hierarchy [EPUB ebook] 

Heavenly Hierarchy by Pseudo-Dionysius is a Pseudo-Dionysian work on angelology, written in Greek and dated to the 5th century AD. It exerted great influence on scholasticism and treats at great length the hierarchies of angels. Excerpt: ‘That every divine illumination, while going forth with love in various ways to the objects of its forethought, remains one. Nor is this all: it also unifies the things illuminated.’