One method in interaction design is to get a clear picture of who is using your product. This book shows who that person is (or was) for SMS. As a mobile app developer, this book helped put a personal face on my market.
I first heard about this book from Trip Hawkins, during a mobile track of the Game Developer Conference. This was before the iPhone came out. While the iPhone may have changed how mobile apps are built and sold, this book remains a classic.
What I learned from PPP was how a teenage Japanese girl began to use her pager and ketai. I learned how she, and others like her, became the social nexus of a new phenomenon. Now, with carriers transitioning from SMS to data plans, this book helps me to ask the question: "who's the leading way" - in a way that might yield a useful answer.