The authors of this book carefully argue for claims they make, backing them up with evidence from carefully conducted experiments (which are all re-described and referenced). They discuss the strength of their claims (which way causation flows and why) as well as the limits of conclusions that can be drawn thus far. They also point to where more research is needed.
The book starts out with (I think a persuasive) argument that happiness can be quantified for purposes of economic study and proceeds to describe what economists have learned by paying attention to happiness. The book continues by exploring new directions of research (such as procedural utility - the idea that not only the outcome matters to people) and implications for policy.
I highly recommend the book to economists or those interested in economics as well as those interested in happiness. I have researched happiness from reading much psychology literature and this book provided new insight that I have not had from the books and articles in psychology.