E H Palmer 
Oriental Mysticism [PDF ebook] 

Originally published 1867. This volume describes not only the basic tenets of the Sufis but also the Ahl i wahdat which was a branch of Sufism. The author’s use of a Persian manuscript treatise by ‘Aziz bin Mohammed Nafasi’ is an indispensable tool, particularly because the author did not merely translate it but gave a clearer and more succinct account of the system. The volume contains an Appendix containing a glossary of allegorical and technical terms in use among Sufiistic writers.