Raw, Laurence
British Council , 1994
Buku Teks
Cummings, William K., author
Springer, 2012
eBooks
Rooijakkers, Ad., author
Grasindo, 2008
Buku Teks
Weimer, Maryellen, author
Jossey-Bass , 2002
Buku Teks
National Institute of Educational Planning and Administration , 1985
Buku Teks