Two conclusions stand out; one surprises me. The unsurprising conclusion is that bishops can and do significantly affect the spiritual vitality of their dioceses, and in certain particular ways that the authors describe. They reach that conclusion partly by a well-argued process of statistical elimination and partly by just talking to bishops and those who work with them.
The surprising conclusion is that the whole spectrum from vital to dismal can be found among the dioceses making up the Church in the United States. Most everybody who is not in denial is familiar with the problems; but in every region of the country, one can find dioceses that are bucking the negative trends. They provide clear, explicable examples of what works, which shows that it's possible to address the problems successfully.
Hence the mystery: why aren't the dioceses that are doing poorly addressing the problems by emulating those that are doing well? Is it primarily that there aren't enough priests who would make the right sort of bishop? Or are other factors, either on their own or in combination, more important?
I wish I had the answer. Not that any bishop would give me a job if I did....