The Sickness Unto Death by Søren Kierkegaard (1849)

You can be busy, well-liked, untroubled, and entirely lost to yourself, and this is the book that will tell you so. Writing as Anti-Climacus, a height of Christian faith he openly placed above his own, Kierkegaard recasts despair as something far stranger than sadness: a quiet misrelation in which the self refuses to become the self it was made to be, refuses to rest in the power that grounds it. He builds the argument as a taxonomy, three forms of despair across three shapes of selfhood, and the unbearable thing is how precisely each one finds you. The despair that does not know it is despair, he insists, is not the gentlest but the deadliest; to feel completely at home in the world is already to have misplaced your soul. And the sickness named in the title cannot do the one thing sickness does. It will not kill. It only keeps you dying, awake to a wound you have spent a lifetime calling contentment.