One, No One and One Hundred Thousand by Luigi Pirandello

The book tells the story of a man who has his identity shattered when his wife casually notes that his nose tilts to the right, something he had never noticed before. This seemingly insignificant comment leads him into an obsessive quest to understand how he is perceived by others, and he gradually loses his sense of self as he fragments into a multitude of characters. The protagonist's existential crisis intensifies as he realizes that everyone he meets perceives him differently, leading him to question his own existence and ultimately, his sanity. The 1419th greatest book of all time