Sufism -- Sufis -- Sufi Orders
In one of the more important works of early Persian literature, Kashf al-mahjub (Uncovering the veiled), Hujwiri, or Data Ganj Bakhsh as he is also known, in the 11th cent. CE discussed various views of Ma'rifah: Gnosis or Direct Knowledge of God, which is an important epistemological principle in Sufism. (Fixed Oct. 20, 2005)
Al-Ghazali's Treatise on Direct Knowledge from God: Introduction indicates the opinion of one of Islam's greatest scholars concerning the possibility that humans--aside from prophets--can attain "direct knowledge" from God.
For more on the unveiling of the unmanifest realm (kashf al-ghayb) see the article On Kashf and Mukashafat, also written by scholars of the as-Sunnah Foundation. The link was down on Oct. 20, 2005, but the page is archived at On Kashf and Mukaashafaat (Spiritual Unveilings)