Tholuck, F. A. G., Ssufismus sive Theosophia Persarum pantheistica (Berlin, 1821).
In Latin. Out of date in some respects, but still worth reading.
Palmer, E. H., Oriental Mysticism (Cambridge, 1867).
A treatise on Persian theosophy, based on a work by Nasafī.
Von Kremer, A., Geschichte der herrschenden Ideen des Islams (Leipzig, 1868), pp. 52–121.
A brilliant sketch of the origin and development of Sūfism.
Goldziher, I., Vorlesungen über den Islam (Heidelberg, 1910), pp. 139–200.
An account of Sūfī asceticism and mysticism by the greatest living authority on Islam.
Goldziher, I., Muhammedanische Studien (Halle, 1888–90), Part ii., pp. 277–378.
Gives full details concerning the worship of Moslem saints.