This is an older book, but one that stands the test of time well. This is the book that spawned the NPS customer service measurement system that many companies are using. If you only read it to get a better understanding of this concept, it is worth the time.
What struck me after reading it is the misconceptions about NPS that people have - it is usually pretty clear who has read the book and who hasn't. The book clearly states how the measurement is an imperfect science, yet those who use it act as if it is beyond reproach.
I highly suggest reading this book to understand how NPS developed and have a better understanding of how to use the data it provides.