Perhaps the greatest living commentator on mysticism in the western (Roman Catholic) Christian tradition is Bernard McGinn of the University of Chicago. He has several books on the topic, mostly from the historical perspective. I’ve read many of these and recommend them highly.
A link to a page listing his books on
Amazon.com:
amazon.com/Bernard-McGinn/e/B001IXMTXE
Christian mysticism is a vast topic, so it is best to pick a spot and jump in rather than attempt to read everything and master the whole field. My favorites include
Sayings of the Desert Fathers and
Life of Anthony (of the Desert),
The Cloud of Unknowing, the writings of St. Teresa of Avila and John of the Cross (if I had to pick one, it would be
The Interior Castle of St. Teresa of Avila), and
The Little Flower of St. Therese of Lisieux. Better yet, learn to read and meditate on the scriptures (
lectio divina), find a good spiritual director, and practice prayer on your own. As a life-long student, however, I do love to keep reading the masters, and the Christian tradition offers lots of help to those journeying on the road.