"So the square root of -1 is...
He didn't press us. On the contrary, he fondly studied our expressions as we mulled over the problem.

"There is no such number," I said at last, sounding rather tentative.

"Yes, there is," he said pointing at his chest. "It's in here. It's the most discreet sort of number, so it never comes out where it can be seen. But it's here."

And here it is indeed, tucked into the heart of a gentle story about several forms of love, with mathematics as a metaphor for life, but also the lantern illuminating its mysteries.

When the Professor's sister-in-law, the Widow, hires the tenth in a string of unsuccessful housekeepers to look after him, this Housekeeper comes into his life knowing only that his memory lasts for just eighty minutes and that she must never bring problems from his little bungalow in the garden to the big house where the Widow lives, but must solve them herself.

Each morning the Housekeeper is met by the Professor at the door with a series of numerical questions (her shoe size, her bicycle registration number), but unlike the other nine before her, this Housekeeper finds dignity in the ritual, understanding why the Professor must go through it. The many notes fastened to his suit jacket, rustling like dry leaves when he walks, don't bother her but rather move her to comprehend his pain of learning anew each day that his full life has been taken from him each time he rises and puts on his coat.

Ms. Ogawa creates moments of quiet beauty throughout the story as the Housekeeper's generous heart, questing intelligence, and open mind meet a handicap of such magnitude with acceptance and compassion. Most of the time love moves gently in this story, as a single mother finds an unusual surrogate father for her son as well as an education for her own curious mind; as Root, her son, comes to love the Professor wholeheartedly; and as the Professor has his circumscribed life infused with unexpected love, expressed and received. The rains fall softly on the blossoms.

But there is another love story here: the subtle root of minus one itself, discreet and yet powerful, lasting. When the thunder roils, the rains lash down and a Shinto temple is blasted by lightning, Yoko Ogawa shows us that certain loves are dangerous wild forces capable of great destruction. She handles this with great delicacy, avoiding with skill what might otherwise have been a cliché.

As the Professor instructs the Housekeeper, "Eternal truths are ultimately invisible, and you won't find them in material things or natural phenomena, or even in human emotions. Mathematics, however, can illuminate them, can give them expression—in fact nothing can prevent it from doing so."

Whether using Euler's formula, the beauty of a prime number like eleven (the book has eleven chapters), or baseball, Ms. Ogawa gives even the least mathematical of us a sense of this illumination as this deceptively simple story unfolds to reveal great beauty at its heart.