An entire generation has grown up since the end of the Cold War, an event that, despite the persistence of Communist governments in China, Vietnam, North Korea, and Cuba, has nonetheless come to seem the manifest destiny of a system so cruel and absurd that one is amazed it lasted so long. The complexity of an episode often reduced to iconic snapshots of revelers atop the Berlin Wall is too easily forgotten, but in REVOLUTION 1989: THE FALL OF THE SOVIET EMPIRE (Pantheon, $30), Victor Sebestyen, who fled Hungary as an infant after the 1956 Soviet invasion, restores the…