The Housekeeper and the Professor by Yoko Ogawa

He is a maths professor with a peculiar problem – ever since a head injury 17 years ago, he has lived with only 80 minutes of short-term memory. She is a housekeeper with a ten-year-old son, who is entrusted to take care of him. Each morning, as they are reintroduced to one another, a strange relationship blossoms between them.