Of the importance of the prophecies of Isaiah, the Bible Dictionary states:
"Isaiah is the most quoted of all the prophets, being more frequently quoted by Jesus, Paul, Peter, and John (in his Revelation) than any other O.T. prophet. Likewise the Book of Mormon and the Doctrine and Covenants quote from Isaiah more than from any other prophet. The Lord told the Nephites that 'great are the words of Isaiah,' and that all things Isaiah spoke of the house of Israel and of the gentiles would be fulfilled (3 Nephi 23:1-3).
"….The reader today has no greater written commentary and guide to understanding Isaiah than the Book of Mormon and the Doctrine and Covenants. As one understands these works better he will understand Isaiah better, and as one understands Isaiah better, he more fully comprehends the mission of the Savior, and the meaning of the covenant that was placed upon Abraham and his seed by which all the families of the earth would be blessed."
Great words cannot be fully appreciated without great effort: "The command to search diligently the words of Isaiah is plainly intended to mean something other than an emphasis upon the necessity of reading them. It implies that without a concerted inquiry one cannot properly understand the words of Isaiah, and ultimately that nothing less than an all-out, investigation will yield the desired results. It means that Isaiah's words are too 'great' to be comprehended by a surface reading only. His book exhibits all the characteristics of a great literary masterpiece and, as such, requires serious effort to be understood. The Savior's recommendation assures us that the knowledge to be gained is worth the effort." (Monte S. Nyman & Charles D. Tate, Jr., Isaiah and the Prophets, p. 128)
Those of us struggling to better understand Isaiah must not allow ourselves to be too discouraged. Nephi loved the words of Isaiah, but readily admitted that he was not easy to understand without a knowledge of the manner of prophesying among the Jews, and, more importantly, without the spirit of prophecy (2 Nephi 25:1-5). Nephi knew that the words of Isaiah would be of great worth to the latter-day saints, but only because we would understand them. Thanks in large part to the Book of Mormon, the latter day saints, he said, would actually comprehend his complex message, for in that day shall they understand them; wherefore, for their good have I written them (2 Nephi 25:8).
Furthermore, Nephi explains that Isaiah is easy to understand in retrospect, in the days that the prophecies of Isaiah shall be fulfilled men shall know of a surety, at the times when they shall come to pass (2 Nephi 25:7). This is certainly true of his Messianic prophecies. Those passages which speak of Christ's mortal sojourn are frequently quoted and clear in their meaning. After the events of the Second Coming transpire, his Millenial prophecies will seem just as clear.
Without the Book of Mormon, we can only imagine how much important Isaiah doctrine we would be missing. Not only would we lack a host of priceless Isaiah commentary, but the many chapters of Isaiah contained in the Book of Mormon seem to be re-recorded because of our weakness. The Lord knew that there would be precious few who would read the Old Testament with the same diligence as they would study the Book of Mormon. His prophets have even had to chide us for our weakness in neglecting this plain and precious witness. Therefore, he included Isaiah's most important chapters and passages in the Book of Mormon for our benefit. If he had not done this, the writings of Isaiah would be just as foreign to most latter-day saints as are the prophecies of Jeremiah, Ezekiel, Zechariah, and others. Thus we see the mercy of the Lord in making provision for us—even in our weakness.
LeGrand Richards
"We are all here today in fulfillment of the words of the prophets, and I love the prophecies of Isaiah because it seems to me that he lived almost more in our day than when he was actually upon the earth, because the Lord gave him to see so many of the things that would transpire in the latter days" (Conference Report, Oct. 1956, p. 23)
Hugh Nibley
"(quoting 3 Nephi 23:1-3.) That quotation alone spares us the trouble of an apology for Isaiah. The book of Isaiah is a tract for our own times; our very aversion to it testifies to its relevance. It is necessary to remind us of its importance, however, because Isaiah's message has not been popular, and he tells us why. The wicked do not like to be told about their faults…says Isaiah, the people of Israel want to hear smooth things: 'Prophesy not unto us right things, speak unto us smooth things.' (Isaiah 30:10.) And ever since, the process of interpreting Isaiah has been one of smoothing him out." (Old Testament and Related Studies, pp. 215-6)