"Mysteries" is a psychological novel revolving around the enigmatic character, Johan Nilsen Nagel, who arrives in a small Norwegian town during the summer. Nagel's eccentric behavior and profound philosophical discussions bewilder the townspeople, as he oscillates between strange actions and profound, insightful observations about humanity. His interactions with the local people, especially his love interest Dagny Kielland, reveal layers of his complex personality. The book delves into themes of existentialism, loneliness, and the inherent mystery of human nature.

The 3683rd greatest book of all time