||Sefer Mordekhai, always referred to as "the Mordekhai," is a gigantic compendium that consists of elaborations on talmudic problems in the style of the tosafot. However, it follows the arrangement of laws used by R. Isaac Alfasi, its aim having been to spread the learning of the French and German scholars and of their predecessors, by attaching them to the work of Alfasi, which had a wide circulation; but the Mordekhai does not refer at all to the content of Alfasi's book. Over 300 books and authors are cited in the Mordekhai, including whole pages from Or Zaru'a and dozens of responsa of R. Meir of Rothenburg in full. The absence of any of the writings which R. Meir of Rothenburg sent to his pupils while he was in prison proves that the book was completed before 1286, the year of R. Meir's incarceration. On the other hand, it is clear from the many references to "my master, Rabbi Mordecai" that the book was not edited by Mordecai himself but by his sons and pupils. If the Sefer ha-Dinim of R. Judah ha-Kohen and Sefer ha-Hokhmah of R. Baruch b. Samuel are still known today, it is almost entirely thanks to the Mordekhai. The history of the spread of the Mordekhai and the transmigrations of its many versions in manuscript and in print is one of the most complicated in all of rabbinic literature. Because of the book's tremendous scope two main compilations of extracts, the "Austrian" and the "Rhenish," were made from it within a few decades, mainly reflecting regional laws and customs, and differing greatly from one another. The Rhenish version - which is the one extant - includes the views of many French and English scholars, and the customs of the German communities. These customs had spread eastward as far as Poland, but were not accepted west of Germany. The Austrian version reflects the minhag of southeastern Europe including the customs of Austria, Hungary, Bohemia, Saxony, and Moravia, and mentions many Austrian scholars. This version was in the possession of R. Israel Isserlein.
In 1376 R. Samuel Schlettstadt edited an abridgement of the Mordekhai (Mordekhai ha-Katan), adding glosses of his own (Haggahot Mordekhai). In print, these appeared independently at the end of the book, but sometimes they were confused with the text. This abridgment was based on the Rhenish version, and when R. Schlettstadt later obtained a copy of the Austrian version, he added some passages from it. The Halakhot Ketannot in the Mordekhai are also R. Schlettstadt's work. Many other abridgments have been made, both by copyists and by printers, this activity having begun, in fact, shortly after R. Mordecai's death. Apart from Schlettstadt's abridgment, there are extant two printed versions of the book and a larger number of versions in manuscript. Many manuscripts are extant in libraries in many parts of the world, but no two of them are identical, and all of them are different from Mordekhai ha-Gadol (the unabridged Mordekhai), also extant in manuscript, which was too long to be copied in full.
The Mordekhai exerted a powerful influence in Germany on the manner of arriving at halakhic rulings until the time of R. Moses Isserles, mainly through R. Israel Isserlein, who relied on it considerably in his Terumat ha-Deshen, and R. Joseph Colon. The book was also most influential in the world of Sephardi halakhah - which it reached in its abridged form - and R. Mordecai b. Hillel ha-Kohen is one of the few Ashkenazi authorities cited by R. Joseph Caro in his Beit Yosef. Many scholars wrote interpretations, amplifications, glosses, or corrections to the Mordekhai. Kizzur Piskei ha-Mordekhai, by R. Joseph Ottolengo, which is generally published together with the Mordekhai also deserves mention. Up to and including the time of R. Moses Isserles, small groups of Jews would get together for the regular and systematic study of the work.
R. Mordecai b.Hillel ha-Kohen (1240?–1298), author and rabbinic authority in Germany. The only biographical details known of him are that he was a descendant of R. Eliezer b. Joel ha-Levi, a relative of R. Asher b. Jehiel, and a brother-in-law of R. Meir ha-Kohen, author of the Haggahot Maimoniyyot, that he was an outstanding pupil of R. Meir b. Baruch of Rothenburg, R. Isaac b. Moses (author of Or Zaru'a), and R. Perez b. Elijah of Corbeil. He appears to have spent some time in Goslar (Resp. Maharam of Rothenburg, ed. Lemberg, 476), from there moving to Nuremberg, where he died a martyr's death in the Rindfleisch massacres, together with his wife and five children.