I have read William Johnston who was a jesuit mystic, and listened to on youtube to a benedictine mystic David Steindl Rast. Also I know about Merton and a few others.
As someone pointed out there is variety what we mean by mystisism. W Johnston descrices how we leave all forms of God to know God within ourselves. He uses a language that may confuse those who aren't experienced of New age concepts. According to Johnston we are to leave all attachments to God and experience God in the unknown. Johnston means Jesus died on the cross so we can find God within ourselves. It's not about Jesus taking the sins of the world on himself, but so we become one with God, or actually be God. He says we have to leave all concepts/images of God to know God.
Here are some quotes from him:
"Contemplation removes the "roots" of sin."
"The aim of contemplation is, then, to return to the state of original justice, before sin, all the faculties of man were united and fixed on God in perfect union with Him"
"And not only is God the being of all things, He is also our being."
The mysticism of the cloud of unknowing - William Johnstone