This is a book for professional theologians and for those non-professionals who are well acquainted with Catholic dogma, the Roman missal and the nature of theological controversy in a Catholic context, and who also have the ability and the patience required to follow a process of sustained, subtle theological reasoning. Excellently translated, this is a brilliant explanation and defense of Canon Masure's own positions on the long controverted questions of the essence of the sacrifice of the Mass and the constitution of the sign of the sacramental insolation it represents. His position is linked by the author to the encyclical on the liturgy, Mediator Dei, which he interprets in favor of his position and as implicitly showing disfavor toward other views formerly prominently held by some. Part I treats of The Essence of the Eucharist and Part II of The Origin of the Eucharist. Undoubtedly a major contribution to the theology of the Mass, it is profound and profitable reading for those both interested and able.