||An anthology and summation of German-Jewish scholarship concerning the laws of Orah Hayyim and Yoreh De'ah down to the Author's own time, basing himself chiefly on the Tur of R. Jacob b. Asher. The aim of the book was threefold: to assemble in one source all the relevant data necessary for establishing halakhah - but omitting the argument; to lay down the halakhah; and to include the new rulings of the latest scholars, such as R. Israel Isserlein, R. Jacob Weil, R. Joseph Colon and, in particular, his own father. The style is that of the other classic anthologists: R. Mordecai b. Hillel, R. Zedekiah Anav, R. Moses b. Jacob of Coucy, and R. Isaac b. Joseph of Corbeil. The arrangement of the material is similar to that of the Tur, on which R. Landau's work was based. The work is distinguished by a discriminating interweaving of varied material taken from a large number of works. The whole is transformed into a complete and unified literary creation. A novel feature in the work is the interlacing of kabbalistic theory in halakhic matter as an aid to arriving at a decision. Another feature is the approbations the author sought from other rabbis, and the Ha-Agur is the first Hebrew book to contain these haskamot. The book was published in a number of editions together with Hazon and the various editions differ because of corrections made by the author. Variations occurred especially in editions of the Hazon which, from the third edition onward, was much abridged. Ha-Agur contains a wealth of information on the teaching of the German scholars of the 14th and 15th centuries and was widely known among scholars and students alike.
R. Jacob Baruch b. Judah Landau (15th century), German talmudist. R. Landau's father, R. Judah (d. 1464), was a favorite pupil of R. Jacob Moellin and a relative of R. Jacob Weil who refers to Judah in his responsa with great respect. R. Judah maintained a large yeshivah in Germany and was renowned as a posek. Toward the end of his long life R. Judah was involved in a vehement halakhic dispute with the two well-known rabbis, R. Judah and Moses Mintz.
Of R. Jacob Landau few biographical details are known—even the dates of his birth and death cannot be established. It is certain, however, that he was born in Germany and educated there, chiefly by his father. He wandered to Italy along with the great wave of German emigration at that time and in Pavia in 1460 met Joseph Colon, whose customs and rulings he quotes frequently. In Pavia he wrote Hazon, a unique work introduced by the words "How could it be?" From Pavia he proceeded to Naples (1487) where he worked as a proofreader in the new Hebrew printing press established there. That year he corrected the proofs of the Book of Psalms with R. David Kimhi's commentary and published at the same press his own main work, Ha-Agur, with the above-mentioned Hazon. Nothing is known of any other books corrected by him and it seems that he ceased proofreading, although it is known that his son Abraham undertook that work at the same publishing house in 1492. R. Landau's father-in-law was R. Abraham Sachs, also a renowned German scholar, who emigrated to Italy. It is also known that R. Jacob Landau was well versed in Kabbalah. R. Landau's reputation rests upon his Ha-Agur.