I ran across the word mien in a book. Is it a typo?
Probably not. Mien is a real word that refers to a person's appearance, or the way someone carries himself or herself. Everyone has a certain demeanor — a characteristic manner that gives insight into an individual's emotions and overall character.
Think about someone you've just met. What does the "air" about your new acquaintance tell you? Is their bearing bold or timid? Are they engaging or aloof? Can you get a sense of what's going on inside their heads by their mannerisms and conduct?
Mien appears in many great literary works, including Robert Louis Stevenson's Dr. Jekyll and Mr. Hyde:
The middle one of the three windows was half-way open; and sitting close beside it, taking the air with an infinite sadness of mien, like some disconsolate prisoner, Utterson saw Dr. Jekyll.