A few months ago, I heard Brett Salkeld (the author) talking on the First Things podcast about this book. I was intrigued, and we ended up selecting it for my Christian reading group.
The overall project is a deep-dive on Catholic theology of transubstantiation and some proposals for Christian unity on the Eucharist. It is fairly high-level and challenging. But if you have any tolerance for academic theology/philosophy, I think it is well worth the investment of time and attention.
Salkeld covers, very roughly, three topics:
(1) He summarizes the historical Christian understanding of the real presence prior to Aquinas. For Catholic apologetic purposes, this is extremely useful, because it helps respond to cherry-picked quotes from certain church fathers that appear to deny real presence. For example, Augustine at times refers to the bread and wine as "signs" or "figures," and the book helps makes sense of this.
(2) He then does a deep-dive on Aquinas to really get to the heart of exactly what is meant by "transubstantiation." This includes a question-by-question walk-through of several of the key passages in the Summa. The analysis is thorough and addresses strange corner-questions, like: "Quid sumit mus?" (What does the mouse eat?), i.e., "if a mouse goes into church after the Mass and sneaks a bite from the tabernacle, does transubstantiation mean that the mouse consumes Christ?"
(3) He then does similar deep-dives into Luther (consubstantialism) and Calvin (instrumentalism) to contrast their views with Aquinas. The conclusion here is that, in many important respects, the reformers were arguing against non-doctrinal perversions of Aquinas's articulation. Salkeld demonstrates how close Luther and Calvin really were to Aquinas's original formulation, and he suggests some ecumenical paths forward for church unity on this issue with our Lutheran and Calvinist brothers and sisters.
Although Salkeld's intention in writing the book is ecumenical, it is worth reading just for the explanation of Catholic doctrine. I'm a cradle Catholic and thought I understood transubstantiation reasonably well, yet the book was honestly transformative of my thinking. In particular, Salkeld highlights treatments of real presence that were near-cannibalistic (i.e., the first confession of Berengarius) and how theology pivoted away from that extreme until Aquinas's precise formulation of transubstantiation.
As I gather from Salkeld, the gist of Aquinas's articulation is (very roughly): transubstantiation is not the removal of the substance of the bread and replacing it with the substance of Christ. Rather, God is the very essence of being, in that all of reality subsists in Him. The consecration at the Mass tears the veil that separates the substance of the bread from the underlying reality of God in which it subsists. Salkeld's detailed explanation of Aquinas and working out of the implications of the doctrine brought my appreciation for the Eucharist to an even higher level.
I also enjoyed the discussion of Luther and Calvin, and the book improved my understanding of their theology. I felt Salkeld did a good job framing their objections to transubstantiation in the context of contemporary theological development and showing how the reformers were closer to Catholic doctrine than we might think. Although, I'll have to leave it to the Lutherans and Calvinists to opine on whether Salkeld's ecumenical outreach is helpful for them.
I really, really loved the book, but in fairness, I'll close with two comments to point out who it is NOT for:
First, although Salkeld does an incredible job explaining Catholic doctrine and tracing historical development, he doesn't do a step-by-step justification of the fundamentals of Catholic theology. For example, there's no chapter with a scriptural analysis of John 6 and the Bread of Life discourse. If you want a book to give you that basic introduction to apologetics, I would suggest Barron's "Eucharist" and Pitre's "Jesus and the Jewish Roots of the Eucharist."
Second, the book does a superb job addressing Catholics, Lutherans, and Calvinists. But one view that does not get much attention is, unfortunately, the one that in my experience is the most dominant in American Protestantism—the theology that takes the Lord's Supper as purely symbolic (whether as official teaching in most evangelical seminaries, or the practical understanding in the pews of most Protestant churches). Because those groups don't have a true sacramental theology, there's not much in the book's scope to address them. So if your goal is ecumenical outreach to "it's just a memorial" evangelicals, Salkeld will need to write another book.