At the moment I\'m reading \"Rules of Play\",
subtitle \"Game design fundamentals\",
672 pages,
by Katie Salen and Eric Zimmerman
(Look for it at mitpress.mit.edu)
This is truly fundamental and basic stuff, so I don\'t bother much reading/buying other stuff while I\'m not through it. Besides it systematic approach, it\'s quite entertaining.
I can only recommend it.