Sun Tzu Art of War is certainly a classic. Most of its concepts revolves around human psychology. The fact that it is still very much applicable in the current world indicates that nothing much have changed for thousands of years in terms human psychology :)

If you like Sun Tzu Art of War, then another book you might find interesting will the Thirty-Six Stratagems. It is a consolidation of 36 essays depicting different strategies and tactics. I find them very relevant in today's world as well. Some of those essays are based on stories from the era of Sun Tzu.