This contribution of Jewish history fits into the classic critical methods that ignore the possiblity that ancient traditions (called Torah SheBaal Peh) could actually exist. If one is interested in critical methodology that is evaluating all extant materials, comparing contrasting and interpreting then one should read this book. It covers questions of conversion to Judaism, the prohibition of intermarriage and origins of matrilineal descent. If one is, however, a believer in 'Emunas Chachamin' and believes in the integrity of the Oral Torah, then this study will frustrate and annoy because the author uses terms like 'rabbinize' that somehow the rabbis changed history formulating normative Judaism.
Much of the book is consistent with the critical historian's view (which was set down many years ago - first quarter of the 20th century- see George Foote Moore's Judaism) that our Judaism, the practices and observances of today were formulated during the major rabbinic periods from 200 bce through the talmudic era of the 5th century. So what the author 'proves' from extant literature that the concept of conversion from another nation did not occur until the 2nd century bce. In other words, Ruth the Moabite was not really a convert but rather by virtue of marrying Boaz (which is a complete negation of Torah SheBaal peh) accepted into the nation of Israel. Or the author opines that the Bible is patrilineal and not matrilineal - it was the rabbis that changed this Biblical tradition to follow the mother. Or there really was never a wide ranging prohibition to intermarriage (Ezra not withstanding) but rather the rabbis widened the prohibition creating a notion of 'us vs. them'
Professor Cohen acknowleges that he does not know the motivations for the rabbis to 'rabbinize' (a fascinating indescript term used when the rabbis adopt, or change something according to the critical historian's thesis) and he never gives the possibility that the rabbis had a standing ancient tradition in their hands. The reason he can not entertain such a thought is because there is no corroborating outside evidence to support such a view or tradition. In other words, for the critical historian the lack of evidence is evidence. In other words, Torah SheBaal Peh is a concept that is 'ahistorical' or anti historical for the critical historian. Any tradition that does not have 'evidence' from the outside is not considered historically worthy. The arrogance of the critical historian comes out in a footnote that cites J.D. Bleich, a very well known accomplished Torah Scholar, labeling his work as "its naive fundamentalism and antihistorical pietism render its conclusions useless for the historian." The true view of the critical historian is that to be a believer in Torah SheBaal peh one need be naive or antihistorical.
From a critical point of view, the author is a fine scholar, very thorough. I have said in another place that sometimes the rules of the critical scholar can blind or restrict his viewpoint precluding that the simple piety of the rabbis can be a viable view. There is not a shred of evidence that the rabbis somehow manufacture changes consciously. The rabbis' point of view that they were holding on to ancient traditions, as the author concludes his book, requires hermeneutics and exegesis, not historical research.
One of the basic differences between Orthodox yeshivos and the Conserivative seminaries is the stress on critical history. The major figures of yeshivos are halakhic adjudicators fluent in the Torah SheBaal Peh with negligent emphisis on history, whereas the Conservative institutions are fundamentally historians fluent in history but not fluent in Torah Shebaal peh or halakhic adjudication.