The brass plates of Laban contained the history of the Jews from the creation down to Lehi's day. It included the Old Testament prophecies written prior to Lehi's day. Even the prophecies of Lehi's contemporary, Jeremiah, were contained on the plates (see 1 Nephi 5:13). Almost all the major Old Testament prophets' prophecies were recorded. The Nephite record would not have included prophecies from Daniel, Ezekiel, Micah, or Malachi. This is hinted at by the ministry of the Savior among the Nephites. At that time, the Lord made sure that the most important scriptures were a part of the Nephite record. This required Him to rehearse some of the Old Testament prophecies which were either of great doctrinal importance or which the Nephites did not have. A careful study of 3 Nephi demonstrates that he mostly quoted Isaiah, Micah, and Malachi.
The plates of brass represented a larger body of literature than our current Old Testament. This is apparent from the statement of Nephi, it (today's Bible) is a record like unto the engravings which are upon the plates of brass, save there are not so many (1 Nephi 13:23). The plates of brass obviously contained a considerable body of literature no longer extant in our Old Testament. It contained the words of many prophets whose message, without the Book of Mormon, would have otherwise been lost to the world. These were men who clearly taught of a Messiah who would come in the flesh to redeem his people. These prophets include Zenock, Zenos, and Neum (see the Book of Mormon Index).