I was browsing at the Catholic gift shop in Pt Pleasant this week and found a gem. Evelyn Underhill's "Practical Mysticism".
This was a peculiar find in a Catholic bookstore because while it is true that she is associated with Christian Mystecism, that particular book is quite secular and doesnt mention Christ or Christianity even once. They also had copies of Julians Showings and the Cloud of Unknowing on the shelf, so I am quite sure that someone there is sympathetic to the mystical approach.
My favorite quote from Evelyn has to be the very simple: "Only the Real can know Reality".