This is a hard book to locate, but it’s well worth tracking down. I first learned of its existence when I saw a copy tucked away in a monastery library, and it took me several months of searching online for a used copy, since it apparently has been out of print for years. But I found my copy, and hopefully you’ll find yours as well. Phyllis Hodgson edited all the Middle English manuscripts associated with The Cloud of Unknowing — an anonymously written manual of instruction in contemplative prayer, probably penned by a Carthusian monk as a work of spiritual direction for a younger novice. Six other, much shorter, treatises are generally believed to be written (or translated) by the same author. The Early English Text Society published the Hodgson editions in two volumes, but those books are themselves hard to find; this edition from Catholic Records Press is the only book I know of that gathers all seven treatises attributed to the Cloud author, in Middle English, in one volume. So it is a wonderful book both for personal devotion and for research.
The Cloud of Unknowing and Related Treatises (Exeter, Devon: Catholic Records Press, 1982)