After quoting from Isaiah 2–14 (2 Nephi 12–24), Nephi provided five additional chapters of his own interpretation of Isaiah. Rather than being a strict commentary, it is more of a Midrash—an explanation of what he saw in those chapters that he wanted his people, and us, to understand. It was not enough for his people to know what Isaiah said on the brass plates; Nephi wanted his people to understand what it meant.
That tells us something more about Nephi, doesn’t it? He was a teacher. He was meticulous about what he did. He was very careful. He understood what Isaiah was saying. If you are patient with both Nephi and Isaiah, there is no better guide to take you through Isaiah than Nephi.
The Book of Isaiah is somewhat of a scrapbook of seemingly disconnected revelations, like the Doctrine and Covenants. Each little section is a separate prophecy. Isaiah did not sit down and write all of this in one sitting. We do not know when all the prophecies of Isaiah were put together and collected. The old Jewish tradition is that they were collected in the days of King Hezekiah and finally put into one collection then, but we do not know how decisions were made for selecting the order of the Isaiah writings.