The mourners’ Kaddish is one of Judaism’s most familiar prayers. But where did the custom to recite it originate? It is a question that has intrigued one of the guests at next month’s Jewish Book Week, Leon Charney, author of The Mystery of the Kaddish. Charney, 67, is one of those men of many parts with a book-length CV. He studied Bible and Talmud at Yeshiva University, New York before financing himself through law school by singing in synagogue — he still has a fine chazan’s voice.