The Housekeeper and the Professor by Yoko Ogawa

Housekeeper is a charming tale set in Japan featuring a single mother, her son, and a brilliant mathematician. The young mother is hired as a housekeeper for the mathematician, who was badly injured in a car accident years earlier. As a result, he can retain memories for only 80 minutes.  The housekeeper has her young son (whom the mathematician nicknames Root for square root) come over after school. The three of them quickly bond and the mathematician introduces the mother and son to the beauty of numbers. It's a sweet, touching story that would be ideal for a book group selection.