Writing an analytical book is not even close to the same thing. It's like the difference between making an iphone and making a youtube review of an iphone.
If you want to learn how to analyze, interpret, and evaluate policy and how to causally identify road blocks to development then you need serious training. Actually doing boots-on-the-ground development work, rather than analyzing what others are doing, is a totally different thing (and the way you asked your question, it seemed like you wanted to do the boots-on-the-ground stuff).
In either case, a PhD in Economics is not the best route. Get an MPA with a focus on technical skills and you'll be better off. Why waste 6 years in grad school when you could be out there making the world a better place by analyzing the efforts of others?