George, Robert Lloyd.
Woodhead Faulkner, 1992
Buku Teks SO
Kemp, Geoffrey
Brookings Institution Press, 1997
Buku Teks
Portland, Or. : F. Cass, 1995
Buku Teks
Taylor, C. Robert
Routledge, 1996
Buku Teks SO
Lockman, Zachary
Cambridge, 2004
Buku Teks SO