If Ed Dean's suggestion ("Foundations of Set Theory" by Fraenkel, Bar-Hillel and Levy) is to heavy for you, have a look at "Intermediate Set Theory" by Drake and Singh. One chapter describes/discusses several different axiomatizations and (Zermelo-Fraenkel, Neumann-Bernays-Gödel, Morse-Kelly, Montague-Scott, Quine, Ackermann).