Ashford, Theodor Askounes
Holt, Rinehart and Winston, 1967
Buku Teks SO
Martin, Alfred N.
Lea & Febiger, 1993
Buku Teks SO
Potter, Merle C.
Prentice-Hall, 1978
Buku Teks SO
Winter, Stephen S.
Harper & Row, 1967
Buku Teks SO
Cambridge University Press, 2003
Buku Referensi