The Housekeeper And The Professor by Yoko Ogawa
He is a brilliant maths professor with a peculiar problem - ever since a traumatic head injury seventeen years ago, he has lived with only eighty minutes of short-term memory.
She is a sensitive but astute young housekeeper who is entrusted to take care of him.
Each morning, as the Professor and the Housekeeper are reintroduced to one another, a strange, beautiful relationship blossoms between them. The Professor may not remember what he had for breakfast, but his mind is still alive with elegant equations from the past. He devises clever maths riddles - based on her shoe size or her birthday - and the numbers reveal a sheltering and poetic world to both the Housekeeper and her ten-year-old son.
With each new equation, the three lost souls forge an affection more mysterious than imaginary numbers, and a bond that runs deeper than memory.