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Ordered roughly by what's most palatable to the modern mind:

Secular non-duality:

  Awake: It's Your Turn -- Angelo DiLullo
  The Power Of Now -- Eckhart Tolle

Buddhism with the focus on mystic praxis:

  Seeing That Frees -- Rob Burbea
  Dependent Origination and Emptiness -- Leah Brasington

Mystic/Orthodox Christianity:

  The Ascetical Homilies of Saint Isaac the Syrian -- St Isaac around 700AD  (one of the best Christian mystics (IMO the best))
  The Complete Mystical Works Of Meister Eckhart -- Dominicon preacher around 1300AD
  The Philokalia -- various authors but St Maximos and Kallistos Angelikoudos are great
  The Mirror of Simple Souls -- Porette around 1300AD as well

Gnostic / Neo-Platonists:

  The Gospel Of Thomas -- The Apocryphal Gospels
  Plato's allegory of the cave (you'll get what he really meant after you have the awakening)
  The Enneads -- Plotinus

If you keep searching you'll discover more over time, they come from different regions, cultures and centuries but all tell the same story. They say Sufi's have some good ones too but I stick to Christian works now days.