Lawrence, Norman; Watts, Joanna; Harrington, Richard; Hope, Sally
Oxford University Press, 1997
Buku Referensi
Edworthy, Niall
Transworls Publishers, 2010
Buku Referensi
edited by Alfred P. Fishman
McGraw-Hill , 1994
Buku Teks SO
Murtagh, John
McGraw-Hill Education, 2013
Buku Teks SO
Schaufeli, Wilmar, 1953-
Routledge, 1998
Buku Teks SO