It’s an excellent book, charting the technical difficulties involved in understanding the foundations of Aquinas’s moral theory and describing some of the various solutions to these difficulties that philosophers and theologians have arrived at in recent times. Here, as a taster, are a couple of paragraphs on how proportionalism misunderstands the notion of “the common good”.
Proportionalism can imagine that the common good justifies harming innocent human beings, then, only by a misperception, by supposing that the good is found not in sharing but in accumulating. To the contrary, the common good protects the innocent, for the common good is nothing other than sharing the good with these innocents. (Page 158).
Proportionalism imagines that we can act for the common good when we kill one person in order to save many others. In reality, by killing we do not act for the common good of any human community; we merely act for an aggregate of individuals. By subordinating one individual to others, we separate him and his good from the community, thereby rupturing the unity found in sharing the good. At most we create a new community, a community of our own, but we break up the community of shared goods that included the individual we kill. Although killing one person can produce a greater aggregate of individual goods, it inevitably does so by excluding a person from the shared good. Ultimately, by killing an innocent person we must be seeking some other good beside the common good. (Page 164).