Elliott, Osborn
Harper & Brothers , 1959
Buku Teks
Ziglar, Zig
Heinemann Asia, 1995
Buku Teks SO
Craig, Robert F.
Van Nostrand Rheinhold, 1974
Buku Teks
Lambe, T. William
John Wiley & Sons, 1969
Buku Teks
Prakash, Shamsher
McGraw-Hill, 1981
Buku Teks