Nephi’s expansion of Isaiah adds content to virtually every line in the first part of Isaiah, and will add little to the end. Each concept is recontextualized and expanded to it Nephi’s purpose of prophesying the coming forth of the words of his people. In this case he begins with the concept of the sealed book, and now provides information as to what is in the sealed book.
The sealed book in the Old Testament is not an unusual theme. In the Old World context, the sealed book is one where the meaning is unavailable. Nephi is reshaping this definition. Because he tells us of the contents, the unavailability of the meaning of the book is no longer the definition of “sealed.” Nephi is presenting a sealed book but as with the rest of his prophecy, a sealed book prophecy customized to fit the events of the introduction of the Book of Mormon. The Book of Mormon will be “sealed” in multiple senses. It will be “sealed” in that the meaning is hidden until it is translated. It will be “sealed” because it has been hidden from the world (note Moroni’s use of “sealed” in the Title page: Written and sealed up, and hid up unto the Lord, that they might not be destroyed—To come forth by the gift and power of God unto the interpretation thereof—Sealed by the hand of Moroni, and hid up unto the Lord, to come forth in due time by way of the Gentile—The interpretation thereof by the gift of God). Finally, it is “sealed” because a portion of the book is physically sealed.
Nephi sees that the plates for the Book of Mormon will be physically sealed. As he describes the sealed section, he gives us the only known description of the contents of the sealed section of the Book of Mormon. It contains a prophetic history of the world from the creation to the end of time. If the sealed section‘s prophetic history is at all comparable in detail to Nephi’s visions that are recorded that very explicit nature of the prophecy is the best reason for sealing the book. Such detail is best in either hindsight to confirm the hand of the Lord, or (as in the case of Nephi) sufficiently removed from the events that the detail will not be able to alter the agency of those who would live in those times. The modern communications make this second condition impossible, and the sealing of the book the most natural solution.