It's about all kinds of neurology patients. It was written about 1970, so a bit of it may seem dated, but the stories are really interesting. The title story is about a guy with proposagnosia, lack of facial recognition. His intellectual function seemed otherwise intact, but he literally could not tell his wife's face from a hat hanging on a hatrack. I find some of Sacks' conclusions may be a bit far-fetched, but his stories are amazing. If you saw the movie Awakenings, that was based on Sacks' work, and also part of Rain Man.