Vidiella, Alex Sanchez
Page One Publishing, 2011
Buku Referensi
Crosbie, Michael J.
Rockport Publishers, 1998
Buku Teks SO
Tobias Waltjen, Walter Pokorny, Thomas Zelger, Karl Tonghele
Springer, 2007
Buku Referensi
Reyes, Elizabeth V.
Periplus, 2000
Buku Teks SO
Beal, Gillian
Periplus, 2003
Buku Teks SO