Ender's Game and especially Ender's Shadow. I hesitate to recommend OSC at all due to his Problematic™ personal politics, especially as a member of the maligned group, but they had a big impact on the way I see The System in which we all live.

Spoiler-free: based on a shared societal belief in a looming existential crisis, a group of young adults attend a military school whose curriculum revolves around a war game with sports-like rules. The System uses the war game to identify for positions of relative prestige those students most willing to interpret the game rules in creative ways, most willing to question assumptions brought with them from the school-world into the game-world, but naïve enough to believe the game is over once they've “graduated” from it. The books explore the many ways in which the “real world” : school-world :: school-world : game-world.