“No one can build you the bridge on which you, and only you, must cross the river of life,” the young Nietzsche wrote as he contemplated what it takes to find oneself. Somehow, this man of stark contradiction, cycling between nihilistic despondency and electric buoyancy along the rim of madness, has managed to inspire some of humanity’s most surefooted spirits — among them, the great German poet, novelist, painter, and Nobel laureate Hermann Hesse (July 2, 1877–August 9, 1962), who…