Isaac the Syrian (also known as Isaac of Ninevah) lived in the 7th century CE as a Christian ascetic and monk, of the Assyrian Church of the East, and is regarded as a saint by the Eastern Orthodox churches.
Born in what is now Qatar on the western shore of the Persian Gulf, Isaac became a monk, before briefly serving as the bishop of Ninevah, in Assyria. He then departed south for ascetic life, living for many years as an anchorite on Mount Matout.
He ended his days as a monk in the monastery of Rabban Shabur, where his reflections on inner spirituality were written down by his fellow monks.
Be at peace with your own soul
then heaven and earth will be at peace with you.
Enter eagerly into the treasure house that is within you,
and you will see the things that are in heaven,
for there is but one single entry to them both.
The ladder that leads to the Kingdom
is hidden within your soul...
Dive into yourself and in your soul you will discover the stairs by which to ascend.
The Ascetical Homilies of St Isaac the Syrian, translated by D.Miller (Boston, Mass, 1984; rev. ed., 2011)
Mystic treatises by Isaac of Nineveh, translated by A.J.Wensinck (Amsterdam: Koninklijke akademie van wetenschappen, 1923; reprinted 1969)
Hilarion Alfeyev, The spiritual world of Isaac the Syrian (Kalamazoo, MI: Cistercian Publications, 2000)
Sebastian P. Brock, St.Isaac of Nineveh 
Sebastian P.Brock, The wisdom of St.Isaac of Nineveh (Kottayam, 1995; Oxford: Fairacres Publications, 1997; Piscataway, NJ: Gorgias Press, 2006)
Patrik Hagman, The Asceticism of Isaac of Nineveh (Oxford: Oxford University Press, 2010)
from Saints, Sufis and Yogis, 3rd ed, v2