This review will contain spoilers!
In many sentences:
11, a prime number, is quite prominent throughout the book. Root turns 11 on the eleventh. The Professor dies 11 years after the Housekeeper worked for him. She is the 11th (counting her first term with the Professor as a separate event) Housekeeper to work for him. There are 11 chapters in the book. I'm not sure if there is some deep significance to this or if Ogawa simply wanted to interlace the book with this particular prime number. Regardless, I thought it was worth mentioning.
Initially what struck me the most about this book was the charming intimacy that it evokes in the reader. The Housekeeper (as the narrator) speaks simply and honestly to the reader. She does not attempt to use extravagant or overly descriptive language. Instead, she presents this simple story about a man who loved Mathematics and cared for the well-being of children. I really enjoyed reading it and found it very touching thinking about how these 2 people could care so much about a man who (for all he knew) could only have an 80 minute relationship with them.
More than anything, though, I enjoyed how much Ogawa made math something that could lead to the flourishing of a relationship. It sounds odd, but that is exactly what happens between the Housekeeper, the Professor, and Root, all thanks to the Professors passion for mathematics. This passion is even contagious to the reader. I (independently) discovered something called Project Euler online that encourages people to learn to program and improve their understanding of mathematics. Between that discovery and the passion for Math present in this book I have found myself wanting to really learn math again.
My only real complaint with the book was the number and frequency of inconsistencies. For example, the Professor is first introduced as being weak and frail, then described as surprisingly strong, then surprisingly weak. I can understand being surprised by being either weak, or strong, but it was annoying to see it happen in both ways. There were several other errors (such as a mathematical one corrected by whoever read the book before me) that unfortunately caused enough of a distraction to annoy me. Otherwise, this is a great book that anyone interested in a touching (or mathematical) book will certainly enjoy.