Although nearing ordination at the Yeshiva University's Rabbi Isaac Elchanan Theological Seminary, he transferred to the Jewish Theological Seminary of America (JTSA), where he was ordained in 1934 and received the advanced Jewish legal degree of Hattarat Hora’ah under the great talmudic scholar Rabbi Professor Louis Ginzberg.
Despite the difficulties facing a congregational Rabbi raising a family, Klein volunteered for the U.S. Army during World War II as a chaplain.
The philosophy upon which A Guide to Jewish Religious Practice is written is stated in the foreword: "The premise on which Torah is based is that all aspects of life - leisure no less than business, worship or rites of passage (birth, bar mitzvah, marriage, divorce, death) - are part of the covenant and mandate under which every Jew is to serve God in everything he does.
In the eyes of Torah there is, strictly speaking, no such thing as the purely private domain, for even in solitude - be it the privacy of the bath or the unconsciousness of sleep - one has the capacity and the duty to serve God."
Rabbi Louis Finkelstein, scholar of the JTSA, wrote: "There are those who would think that we have but two alternatives, to reject or to accept the law, but in either case to treat it as a dead letter.
Meeting minutes, annual reports, bulletins, and sermons relating to Klein's rabbinical vocations in Springfield, Massachusetts and Buffalo, New York are also included.
The papers contain photographs, wartime letters, and military records of Klein documenting his service in World War II as a director of Jewish religious affairs in Germany.