This is an excellent book, that brings their research - and that of others - to the intelligent but not expert reader. (Think: broadsheet newspaper reader)
I am an economist (I teach at LSE), but I am not a development economist. I have no vested interested in the area. I found this a straightforward read - 2 days worth of holiday reading. I think it spot on for the target market - my wife is currently reading it.
The conclusion are broad: poor people are rational, but often ill informed, and that becoming well-informed takes time and effort. As a result, unless everyone understands what the poor think, and why they think it, policies may not work. If poor people don't believe immunisation works, they won't want it whether it is free or not. If poor people think that education is only worthwhile for the brightest, they won't send their kids to school unless they think that they are bright. And if teachers have the same views, their efforts in teaching weaker students will be weak, and universal education will not achieve much. In contrast if schools and parents believe in education, universal education will work much better, for any given level of staffing, funding, etc. We therefore need to understand - and sometimes work to change - beliefs.
The authors are great fans of "random controlled experiments" whereby policy is applied to one group and not to another, and the results compared. This is obviously a good idea, but it would be nice to know a bit more about whether the results are replicable. After all, if beliefs matter, results from one place in India may not travel to another in India, let alone to Africa, etc.
I make two mild criticisms. The books intellectual "straw men" (Jeff Sachs and Bill Easterly) are very American. British academic Paul Collier (Bottom Billion - a great book) gets the odd mention, but Sachs and Easterly are the reference points. I am not sure that they are the best reference points, although they are big names people are likely to have heard of.
Finally, some historical awareness would make this a better book still. For example, the authors argue that human capital, laws, etc are useful when the growth spark arrives - which sounds like a lot of the "why england, why not France?" and "Why Europe, why not China?" economic history literature. Similarly their (surely correct) argument that micro businesses and self employment are what poor people do when they can't get steady employment is matched by studies of the Great Depression (or street vendors in Greece today, for that matter). Such parallels would also make us understand that poor economics has a lot in common with the economics of poor people in history, and the economics of poor people in rich countries.