Bottherill, George
Cambridge University Press, 1999
Buku Teks SO
Khan, Hazrat Inayat
The Sufi International Headquarter's, 1956
Buku Teks SO
Copenhagen : C.W.K. Gleeup Lund
Majalah, Jurnal, Buletin
edited by Peter J. Burke
Stanford University Press, 2006
Buku Teks SO
Lisa Osbeck
Cambridge University Press, 2018
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