While Lehi may have known of the existence of the brass plates, these verses hint that he was not very familiar with them. After receiving the plates, Lehi searches them, and gives a description. The nature of the description tells us something about Lehi's religious/historical knowledge, and something about the plates themselves.
While the words we read come from Nephi, it can be surmised that Nephi is either working from Lehi's record in this instance, or that Nephi was present as his father examined the plates. The description of the contents of the plates indicates both what was on them, and the points which Lehi/Nephi thought were important. Thus we understand from verse 11 that the brass plates held the first five books of Moses, including the Genesis account of Adam and Eve. Clearly that was an expectation, and there is little elaboration of the information. This indicates not only that the plates contained the texts, but that it was not surprising that they do so. When neither Lehi nor Nephi explains any more of the books of Moses or of Adam and Eve, it can be presumed that they were familiar with those texts. Thus, while they may not have been intimately familiar with the brass plates, they were familiar with the scriptures, simply from a different source. It would appear that the version of the scriptures used in Lehi's regular worship were something other than the brass plates, though obviously the brass plates contained much of the same information.
The contents of the brass plates themselves suggest that while they are clearly related to our received text for the Old Testament, there are some differences. While there is scholarly disagreement on the precise way in which the Old Testament texts came into being, there is general agreement, and excellent evidence, to indicate that the Old Testament, like the New Testament, is a fortuitous collection of manuscripts, which are miraculous in their preservation, while at the same time unfortunate in those which were lost. The description of the brass plates appears to place them in the category of a set of texts which were of a different composition than the Masoretic text of the Old Testament upon which our modern versions are based.
John Sorenson analyzed the makeup of the brass plates, and suggests that they are a set of scripture which is more oriented to the Northern Kingdom, as opposed to the Masoretic text, which is more representative of the Southern Kingdom:
"Book of Mormon writers mention five prophets whose words appear in the brass plates: Zenos, Zenock, Ezias, Isaiah, and Neum (the last might be Nahum). Of the first four only Isaiah is surely known from existing biblical texts. Internal evidence suggests a reason why: all four direct a great deal of attention to the Northern Kingdom. Since the Masoretic text, which lies behind our King James' version, came out the South, omission of the three of the four (or four of the five, counting Neum) is explicable. Zenos is quoted as saying, "And as for those who are at Jerusalem... " (1 Nephi 19:13). Nowhere else in the extensive quotes from Zenos does he mention Judah or Jerusalem. This in context strongly suggests that he was not located in the territory of Judah. (It is implied in 3 Nephi 11:16 that Zenos and Zenock were of a Joseph tribe, although nothing is said of the location.) The reference to Jerusalem implies a date after David's capture of the city and quite probably after the division of the monarchy (about 922 B.C.). Careful reading of the allegory of the olive tree, from Zenos, as well as Alma 33:3-17 concerning both Zenos an Zenock further confirms a context of a sinful Israel more reminiscent of the time of Amos (mid-8th century B.C. ) than earlier or later. Moreover, Zenock was said to be a "prophet of old," (Alma 33:17) a chronological term not used regarding Jeremiah or even Isaiah. The probability is high, therefore, that the prophets cited from the brass plates date between 900 B.C. and the end of the Northern Kingdom in 721 B.C." (Sorenson, John L. "The "Brass Plates" and LDS Biblical Scholarship" undated mss, p.5).