Evagrius Ponticus

Evagrius is the desert father who wrote the manual. He took what the illiterate hermits of Egypt had learned by experience and turned it into a precise psychology of the spiritual life — the eight 'thoughts' that disturb the soul, which Gregory the Great later reworked into the seven deadly sins. Every later Christian writer on prayer and contemplation, East and West, is downstream of Evagrius. He was condemned posthumously for Origenist views, so his works circulated under other names for centuries (Cassian's Conferences are basically Evagrius for Latin readers). Read the Praktikos. It's short and devastating about how the mind actually behaves when you try to pray.