Royce, James E.
McGraw-Hill, 1961
Buku Teks
Bottherill, George, author
Cambridge University Press, 1999
Buku Teks
Khan, Hazrat Inayat
The Sufi International Headquarter's, 1956
Buku Teks
Copenhagen : C.W.K. Gleeup Lund
Majalah, Jurnal, Buletin
editor, Cynthia Macdonald and Garaham Macdonald
Blackwell Publishers, Ltd., 1995
Buku Teks