This is also one of my favourite books of all time, and many of Dr Sacks' books are other top contenders. Quite apart from his wonderful human aspect, and the fascination and empathy he has with his patients, it's very thought provoking. I had never really considered how totally wonderful our perception and consciousness mechanisms are until I was forced to by reading his accounts of patients who for one reason or another are missing them. For example, the title story is a man whose vision is perfect, but totally lacks the ability to synthesise the forms he's perceiving into objects, so he has no idea what he's looking at. Truly fascinating. What amazing abilities we all take for granted.