Although Nephi says here that he had quoted and taught things from the Torah written in the books of Moses, he preferred to quote from Isaiah, and he particularly encouraged his family members and his readers to "liken all scriptures unto" themselves. Thus, for example, Nephi quoted a prophecy of Isaiah mentioning the "the isles" of the sea (Isaiah 49:1; 1 Nephi 21:1), and we can see how they would have seen the word "isles" pertinent to themselves. For, at the end of 1 Nephi, at the time, Lehi’s group had just recently arrived in the New World. Lehi had not died yet. They all must have been relieved to have arrived safely in the land of promise. They were rejoicing, and they now read in Isaiah that God will not forget even the people who are on the isles of the sea. Nephi knew that he was on land, but how would he have known then how big a land mass they were on? A few years later, Jacob would say, "we are upon an isle of the sea" (2 Nephi 10:20). They must have known by then that they were on a big island, but they had little idea yet that it was continental in scale. But there were also prophecies of Zenos (1 Nephi 19:16) about people on the islands of the sea, and Nephi could see how the words of the prophets Zenos and Isaiah could be likened to themselves. In these descriptions of how the Lord would remember the people of the isles of the sea, we can imagine how reassured the Nephites would have been on finding those prophetic words. We too can see how these great prophesies speak both about us and to us.
The next two chapters, 1 Nephi 20 and 21, are thus quotations by Nephi of Isaiah 48 and 49. Let me emphasize how accurate, how useful, and how really astonishing the words of Isaiah are in the Book of Mormon, and also how intricate and verbally detailed Nephi’s understanding of Isaiah is.
I have looked a fair amount at what people were saying about Isaiah in Bible commentaries being written in the early 19th century, just to compare how they were interpreting these chapters. Actually, it appears that very little was being said in Joseph Smith’s day about Isaiah, and what they were saying was quite sparse, not systematic, and not very sophisticated. For example, in the 18th and early 19th centuries, the dominant reading for the passage in Isaiah about how kings and queens would be nursing mothers and fathers to the gentiles used these words to justify the divine right of kings and to support the monarch’s privilege to speak authoritatively and to give the people an official state religion. Thus, as a divinely favored ruler, the king of England was seen as the nursing father of his people, and Isaiah’s words were seen as justifying the Church of England. In America, Isaiah was likewise used to show that the church should be part of politics and government. The famous French visitor de Tocqueville once said it was impossible for a democracy to be a solid and good government and that America would never succeed. The Americans answered back, confidently, that democracy would succeed because there will be public virtue, and public virtue will be taught by the churches of our states and of this new nation, seeing this passage in Isaiah for local political purposes, to say that religion would become the nursing strength of the society.
But the Book of Mormon does not see this passage in that way. It talks instead about how the gospel will go into the hands of the Gentiles, but that the Gentiles will be nursing fathers and mothers, who will someday bring the gospel back to descendants of Lehi’s people. Ironically, the Gentiles will help reestablish the house of Israel, fulfilling the covenants of the Lord. As obvious as I think this interpretation is in the text, that didn’t seem so obvious to biblical scholars in the 19th century. But careful readers today see that this is what Isaiah must have meant. In this, Joseph Smith was ahead of his time, precisely because he had Nephi’s help, and Nephi understood the words of Isaiah. Nephi grew up in Jerusalem, where Isaiah had lived. Isaiah ended his main prophecies around 701 BC, and Nephi left Jerusalem around 600 BC, so, there was only about a hundred years, only a couple generations, between them. Thus, when Joseph Smith quotes Isaiah at length, we can appreciate that Isaiah was the most important and venerated prophet of Lehi’s time. This may offer yet another explanation of why Nephi used Isaiah so extensively and authoritatively.
In reading the words of Isaiah, Nephi saw particular words and phrases as being (1) relevant to his own world—we did "liken all scriptures unto us" (1 Nephi 19:23), temporally (1 Nephi 22:1, 3, 27). (2) In addition, Nephi relates Isaiah’s prophecies to his own prophetic world view, informed by Nephi’s great vision in 1 Nephi 11-14. I think this is what Nephi means when he speaks of reading these things not only temporally but also spiritually (1 Nephi 22:1-3). And finally, (3) Nephi expects us and all of his readers to see these fruitful texts as being relevant to themselves and to read them "for our profit and learning" (1 Nephi 19:23). Getting through Isaiah is difficult, but it works better when readers try to get things out of Isaiah, not just to get through it. I recommend following Nephi’s model. By pondering each verse of Isaiah in his three ways has yielded rich rewards for me.
See several chapters in Donald W. Parry and John W. Welch, Isaiah in the Book of Mormon (Provo, UT: FARMS, 1998), notably John W. Welch, "Getting through Isaiah with the Help of the Nephite Prophetic View," in Isaiah in the Book of Mormon, ed. Donald W. Parry and John W. Welch (Provo, Utah: FARMS, 1998), 19–45.