Akaruese, Lucky
Xlibris Corporation, 2012
Buku Teks SO
edited by Eric Delson, Ian Tattersall, John A. Van Couvering and Alison S. Brokks
Garland Publishing, 2000
Buku Referensi
edited by Lori D. Hager
Routledge, 1997
Buku Teks
Langdon, John H.
Springer International Publishing, 2016
eBooks
Pawson, Ivan G.
John Wiley & Sons, 1977
Buku Teks SO