Shea writes primarily for Protestants considering conversion to Catholicism - this book is no exception. The book itself is divided into two parts: 1 provides a history of salvation as presented through the six covenants (Adam, Noah, Abraham, Moses, David, Christ) 2 provides an introduction to the traditional four senses of Scripture (literal, allelgorical, moral and anagogical). The material is presented in a deliberately casual manner. At times the language is delightful and effective ("Bullwinkle syndrome"), at other times it approximately the padding of oral speech, occasionally it verges on an insult to the readers' intelligence. Put another way, Shea knows the audience that he intends to reach and writes specifically for them.
For this same reason, the book contains comparatively little detail on its subject. Rather, it presents a commonsense argument for the validity of the approach, building the case in small increments few could deny and providing multiple instances to support each incremental argument. This approach works well for individuals with little knowledge of the Bible or with a bias against the traditional form of interpretation.
In short, Shea does an excellent job of reaching a very specific audience. If you fall within that audience, I recommend the book. If you do not, there are more informative texts available.
Mark Shea gives us a very excellent method for reading the Bible. This book also contains an easy to understand, simplified explaination of the six covenants God has made with man through the ages. It really makes the Old Testament easy to understand and relevant.