I often credit the book Normal Accidents by Charles Perrow as changing my career. I knew that I wanted to teach mathematics for a very long time, but this didn't have anything to do with math. It was a book about the properties of systems that caused them to malfunction. It took a while to assimilate because this wasn't what I was trained in, but once I started looking at the role of systems in day-to-day occurrences instead of treating every event as a local one it opened my eyes to how to effect change that is much more long-lasting. I have spent most of my career in trying to effect change, whether it be in my department, in my university's management, its curricular reform, or even my professional society. I'm not sure if something else wouldn't have come along and triggered that response in me if I hadn't read Perrow's book but it is a bend in my career that is easy to identify.