The paradigm he sets up is potentially interesting: that on the one hand, you might find a "closed" political catholicism where Catholics all join a Catholic party, are only in Catholic civil society organisations (including trade unions) and where the Church regularly intervenes in politics; on the other, you might find an "open" Catholicism where Catholics are no more or less likely to join particular parties than anyone else, there is no specifically Catholic civil society, and the Church is silent. Neither of these has ever actually happened in reality. Continental Europe on the whole veered closer to "closed" Catholicism than the Anglosphere, and much more in the years immediately before and after the second world war than earlier or later, but there are exceptions all along the way (and in an appendix he looks at Malta, which had strong clerical intervention in politics but was otherwise much more "open").
I can see why the book did not do well though. By the time it was published, in 1981, Catholicism was changing out of all recognition; I think there would have been much more in common in what Catholics did and thought between 1950 and 1850 than between 1950 and 1980. There are lots of nice numbers of election results, censuses and opinion polls to try and quantify who thought of themselves as Catholics and who they voted for. But the economic aspect is largely omitted; surely the question of why Catholics tend(ed) to vote more leftish in some countries and more rightish in others can be answered to an extent by how well off they are/were? And concentrating on the numbers alone, useful though they are, means losing focus on the actual topics of debate, which are mentioned only really in passing.
The most interesting question raised but not really answered in the book is to what extent the Catholic Church as a whole, or regional elements within it, ever really aspired to restore the total control of society that the church liked to think that it enjoyed in the middle ages. The evidence from the book is, not very much, and not very successfully to the extent that this was so (with occasional exceptions). That then would lead on to a potentially much more interesting discussion about what the Church can reasonably think it is doing as a political actor at all. But for that one would have to look elsewhere.