As the object of such a perilous mission, of course Lehi would carefully examine the plates of brass. Modern readers get their best description of those plates in these verses. We learn that they contain the Torah, the essential five books of Moses. They also include at least some of the books that the Jewish collection of sacred books calls the histories. Finally, it includes at least some of the books of the prophets. We learn later that they contain two prophets not mentioned in the Old Testament as we have received it; Zenos and Zenock.
How are there books that we do not have? We have lived with the scriptures in their current format for so long that it is easy to assume that they have always been that way. That is certainly not the case. Before they were officially collected into a bible (a word derived from the Greek plural word for books), they were individual books. Some of the preservation of scripture depended upon what preservations different communities might have made.
John L. Sorenson has suggested that there are elements of what we can learn of the plates of brass that point to an origin in the northern tribes, those lost in the Assyrian invasion one hundred years before Lehi’s call. The plates of brass may have contained records of prophets from the northern kingdom, whose teachings were never recorded in the southern kingdom. Our Old Testament is primarily a southern kingdom document.
Even if it had originated in the northern kingdom and had been brought south by those who fled the Assyrians, it was still a living document. Someone was adding to it, as it also contained records up to the reign of Zedekiah, the king at the time that Lehi left Jerusalem.