Gli Ambasciatori by Henry James

Lambert Strether, a respectable middle-aged American, is dispatched to Paris to bring back a young expatriate heir for a backwoods patron, but as he drifts into the city's social life and forms a subtle friendship with an insightful American expatriate, he finds his narrow convictions unsettled; through encounters with art, romance, and ambiguous morals he undergoes a quiet but profound reappraisal of duty, illusion, and what it means to live fully.