Menachem ben Jacob was a tremendously influential Spanish Jew during the rise of Spanish Jewry. He was a Jewish lexicographer and poet who composed the first Hebrew-language dictionary, a lexicon of the Bible; earlier biblical dictionaries were written in Arabic and translated into Hebrew. The father of Hisdai ibn Shaprut was his patron. With the death of Hisdai's father, he made Menachem his secretary. Menachem was a gifted writer. He composed the letter which Hisdai sent to the king of the Khazars. He wrote beautiful eulogies for Hisdai ibn Shaprut's parents.
He is most famous, however, for his biblical dictionary, called the Machberet. Written in Hebrew (instead of Arabic like most scholarly texts written in Muslim Spain and Babylonia), Menachem's dictionary was available to the Jews of Christian Europe as well as the Jews of Spain. We take understanding of Hebrew grammar, dictionaries and translations of sacred texts for granted. Back in the 10th century, a biblical dictionary was an extraordinary gift to the Jewish world. Scholars didn't yet understand the grammatical structure of Hebrew. (It wasn't until one of Menachem ben Jacob's students figured out that Hebrew verbs consisted of three root letters that any real head-way was made in Hebrew grammar). Written by hand on parchment, the Machberet text was an instant hit in Europe. His dictionary was not without controversy, however. Shortly after the Machberet appeared, it was vehemently attacked by Dunash ben Labrat, who claimed that certain definitions were likely to lead the reader to erroneous interpretations of halachah and belief. The expectation that the dictionary would therefore become a source of heresy explains the bitterness of the attack. As a result of Dunash's responsum, Hisdai ibn Shaprut fired Menachem from his post as secretary.
Menachem himself did not reply to Dunash's criticisms, but three of Menachem's pupils, Isaac ibn Kapron, Isaac ibn Gikatilla, and Judah Hayyuj, took it upon themselves to defend their master.
Although he made mistaken assumptions about the nature of the Hebrew three-letter root, Menachem ben Jacob paved the way for his students to do greater research into the development of the Hebrew language.
Rashi used Menachem ben Jacob's Machberet extensively as did Rabbenu Tam.
Sources: Gates to Jewish Heritage