The Death of Ivan Ilyich
Leo Tolstoy
Tolstoy gives his protagonist a bruise from hanging curtains and turns it into a meditation on mortality, self-deception, and the terror of dying badly. Ivan spends his final weeks realising he has never lived. The servant Gerasim, who is simply kind, becomes the most devastating character in the book. It is the shortest long novel about the cost of never having asked what you wanted from life.