In this latest addition to his series on Jewish observances, Greenfeld has set himself the unpromising task of explaining an occasion without a clear-cut rationale or origin which, moreover, is marked differently by the three branches of Judaism. The result is not a book for a child much below the bar mitzvah age--and one might wonder how much interest inheres, even then, in learning of the various textual bases (in the Midrash and in separate parts of the Talmud) for the ""belief that a Jew is ready to assume his duties as a Jew following his thirteenth birthday."" To most, that seems a natural time for a boy's coming-of-age (something Greenfeld himself appears to recognize when he notes, at the close, that the bat mitzvah ""usually takes place on a girl's twelfth birthday, a recognition of the fact that girls mature at an earlier age than do boys""). There follows a description of the boy's new privileges and obligations--but no sooner have we learned all about wearing the tallit (a special fringed shawl) and putting on the tefillin (the two boxes bound to forehead and left arm) for morning prayer, than we hear--along with other divergences in observation--that ""neither is part of a Reform Jew's coming of age."" Finally, ""the actual ceremony differs among the three movements of Judaism""-though certain common elements can be cited. Greenfeld, who begins with a circumspect reference to ""extravagance,"" devotes only a few words, fittingly, to the party-aspect. Overall, he is as unponderously informative as in the earlier books--with the difference that much of the information is either extraneous or not universally applicable. . . and little, therefore, bears directly on the experience of many young people. The illustrations--grave but not grim--are largely confined to framed heads.