Superb review of Judaism in the rabbinic period, and a must-read for anybody interested in this subject, particularly the interface between Hellenism and Judaism. Cohen's treatment is technical but highly accessible and readable. His chapter on Judaism as an "ethnos" is excellent, as is his treatment of "Ioudaios, Iudaeus, Judaean, Jew".
This is an excellent book that reviews the historical origin of the concept of jewishness between the first century b.C. and the second century C.e. The author's thesis is that jewishness is a psychological category and does not really correspond to any identifiable combination of religion, ethnicity, nationality and so on. Up to the II century C.e. the greek term Ioudaios indicated essentially a nationality (that of people born in the region of Judea) or an ancestry (that of people whose parents of grandparents were born in the region of Judea). After that period, however, a shift is observed towards a meaning more linked to religion and ethnicity. The author demonstrates that, as a consequence of the multiple meanings attributed to the term Ioudaios, many real, historical persons could be thought both to have been and not to have been Jew; the most striking of these is the Herod the Great, the last king of Judea. A very interesting complement to this book is S. Sands' "The invention of the Jewish people" (needless to say both authors are themselves Jew, whatever meaning they attribute to this word). It is relevant to remark that a group of people needs not be purely "psychological": biologically defined ethnic groups exist which can be recognized as groups (because of a peculiar distribution of the frequencies of some alleles - gene variants), even though in human populations they are never homogeneous enough to allow their members to be unequivocally identified (i.e. the group as a whole exist but its members do not individually differ from the rest of the world population enough to be recognized). This is perhaps a complicated concept that is not properly stated in either book: if the ethnic group is defined on the basis of the peculiar frequencies of the alleles of its members, we cannot expect any of its members to possess the frequency characteristic of the group: every member as just two alleles of each gene. A merit of both books, however, is that they completely rule out any biological pseudo-explanation, while stressing the historical and psychological factors of the identity of the Jews.
Absolutely brilliant book. If you are interested in the subject, I cannot recommend it highly enough. Easy to follow (the author states his aims right from the start) and virtually puts you right there. Marvelous book