Leahy's book surveys the phenomenon, especially postwar and post-Vatican II, of "movements" in the Roman Catholic Church, many of them communitarian in nature. Some have been founded by clergy, but a goodly number originated in the charisms of lay Catholics. It is this phenomenon of charism on which Leahy puts a good deal of stress.
After an introductory section defining the subject and its importance, Leahy divides his book into three parts, one on "The Emergence of Ecclesial Movements," a second on "Reading the Phenomenon" (relating the movements to the council, to charisms, to apostolic sucession, to evangelization, and to Mary), and a third on "Considerations and Perspectives," examining some issues of how movements fit into existing church structures. There are footnotes and a bibliography of significant length for such a short book.
It is difficult to assess the usefulness of the book or its intended audience. It gives an overview of the phenomenon of church movements, but it seems to skim the surface and to consist mainly of quotations from authoritative church sources, especially popes. The only chapter in which I sensed the author's engagement was that on the role of priests, and consequently it was the only chapter that really held my attention. A variety of movements are named, but there is scarcely any detail about their individual character and their daily lives. Again, a single example of the impact of the Alpha course on an individual life lit up the page, but that was soon past. I was also surprised that the author gives so much space to Pope Benedict XVI's enthusiastic support for movements, yet fails even to mention the movement to which the Pope is closest, and has been for many years, the Integrierte Gemeinde of Munich.
In short, Ecclesial Movements and Communities is a well of information and a pointer to what many churchmen (and a few women) have said on the subject, but is itself not a very engaging read. I might suggest it to a student as a reference, but I would not assign it for class reading.