From around 800 the term Sufi (from the Persian for coarse wool, denoting the kind of garment worn) was applied to Islamic mystics who adopted ascetic practices as a means of achieving union with God. The philosophers al-Ghazali and al-Farabi practised Sufism and contributed to its mystical doctrines. See also illuminationism.
http://www.bbc.co.uk/religion/religions/islam/subdivisions/sufism_1.shtml An outline of the history and practice of Sufism
http://theamericanmuslim.org/tam.php/features/articles/sufism_an_annotated_resource_guide/ An annotated bibliography on Sufism