I found this to be fascinating, but not for reasons that would apply to most people. This book, you see, concerns the discovery of Mussar -- which is more or less a Jewish approach to ethical matters -- by an unobservant Jew, who eventually has a series of meetings with a rabbi in New York in which he learns more about it.
The rabbi in question is the head of the high school and seminary where I studied for eight years. Seeing my school through the eyes of an outsider? Utterly fascinating.
Otherwise... not a bad introduction to Mussar. Kinda simplified, and definitely eclectic, but not a bad read, and I suppose it might be a useful starting point, which is all it really tries to be.