This book is one of the best methodsbooks written! It uses simple tecniques like regression and real life examples to show how difficult it is to draw the right conclusions from the data and the numbers. The examples given are connected to the effects of policy changes, and over and over, we see how conclusions that seem sensible, are actually erroneous.
I have a couple of meters with books on statistics and social science methodology, because I have been teaching this at University for some years. If I had to pick two books only, this would be the first one, as this shows you what you can USE the analysis for. The second book would be a more technical book that explains the statistical part, but they are numerous, and I do not have just one favorite.
If you are a policymaker this book is invaluable, as it teaches you to ask the right questions. It does not matter how good the numbers look if they are not the right numbers!