It is important to understand that the next four books, Jacob, Enos, Jarom, and Onmi, are all written upon the small plates that Nephi made thirty years after leaving Jerusalem (2 Nephi 5:30-32). Jacob did not make his own plates—he recorded his prophecies on the plates that Nephi had made, These plates are called the plates of Jacob, and they were made by the hand of Nephi (Jacob 3:14). These plates were filled up by the last of these men, Omni, who said, these plates are full. And I make and end of my speaking (Omni 1:30).
The fact that Nephi entrusted the plates with Jacob shows the high regard that Nephi had for the prophecies and integrity of his younger brother. These plates were designed only for spiritual things, considered to be most precious (v. 2).