Orthodox Christianity has a very rich contemplative history. In particular, understanding concepts like the nous do quite a bit to illuminate our more modern idea of ego and the false dialectic of big vs. small / null ego.
- Byzantine Theology is a good overview of many of the concepts that differentiate the East from the West (https://www.amazon.com/Byzantine-Theology-Historical-Trends-...)
- Divine Essence and Divine Energies (a critical distinction that has been lost in the West) (https://www.amazon.com/Divine-Essence-Energies-Ecumenical-Re...)
- Any of the works by St. Maximus the Confessor
Lots more...