A light but enjoyable read that scatters numbers and facts about the brain rather like The Curious Incident of the Dog in the Night-Time (http://www.goodreads.com/review/show/23326297). However, in other respects, it's very different, being set in Japan and being primarily about friendship.
The eponymous housekeeper is a young single mother (herself the only child of a single mother) with a ten-year-old son. She becomes daily housekeeper to a former maths professor whose head injury in 1975 means he only remembers the most recent 80 minutes, plus things before 1975, nearly 20 years before the story is set (~1992).
Numbers are now the professor's life; he works on problems for magazine competitions, and he comes alive when he spots numbers or patterns to explain to his increasingly interested housekeeper. When he discovers she has a son, he is adamant that he must come to the house after school and in the holidays: he adores children, and thinks their needs (or his exaggerated perception of them) trump everything else. Thus a relationship is built between the prof and the boy (nicknamed Root), based on numbers and baseball statistics. His short memory span makes him an incredibly patient teacher.
The practicalities and humour of coping with the prof's condition are well portrayed, and the relationships are very touching. And yet, despite the efforts of the housekeeper and her son, the professor's capacity for joy is literally limited. Is there much point having any sort of friendship with or giving happiness to someone who will not remember it? The housekeeper and her son learn so much from the professor, but does he get anything meaningful in return?
Despite all the notes, he has to start each day, each situation, anew. (The hire and fire nature of a housekeeping agency has parallels.)
Some of the maths may be a little obscure for some readers, but not fully understanding it shouldn't impair enjoyment of the book. The prof's message is not about right answers, but listening to and feeling numbers, and there are times when the passion borders on religious, "I needed this eternal truth... I needed the sense that this invisible world was somehow propped up by the visible one... Somehow this line would help me find peace."