Victor L. Ludlow explains the authorship of Isaiah.
Victor Ludlow, "Isaiah: Authorship," in Encyclopedia of Mormonism, ed. Daniel H. Ludlow (New York: Macmillan), 1:699–700
Of the writings in the Old Testament, the message of Isaiah enjoys high priority among Latter-day Saints. The attraction derives primarily from the extensive use of Isaiah in the Book of Mormon. Secondarily, chapter 11 of Isaiah was quoted to Joseph Smith in a vision in his earliest days as a prophet (JS-H 1:40) and became the subject of a section in the Doctrine and Covenants (D&C 113). In addition, Jesus Christ has given revelations about, and prophets and apostles of the latter days have frequently quoted from and commented upon Isaiah's words when instructing the Saints.
Traditionally, the book of Isaiah has been ascribed to a prophet living in the kingdom of Judah between 740 and 690 B.C. In Germany during the late 1700s, several scholars challenged this view, claiming that chapters 40-66 were written by one or more other individuals as late as 400 B.C. because of the specific references to events that occurred after Isaiah's death. This outlook now permeates many Bible commentaries and has led to the postulation of a second prophetic writer who is commonly called in scholarly literature "Deutero-Isaiah." Indeed, a wide variety of theories regarding the date and authorship of Isaiah now exist. However, LDS belief in revelation and the seership of prophets, along with the quotations from Isaiah in the Book of Mormon and its admonitions to study his writings, have reinforced Latter-day Saints in the traditional view concerning the date and authorship of Isaiah, in the following ways.
First, while some scholars argue that prophets could not see the future and that, therefore, the later chapters of Isaiah must have been written after Isaiah's time (e.g., Isa. 45 concerning Cyrus), Latter-day Saints recognize that prophets can see and prophesy about the future. In chapters 40-66, Isaiah prophesies of the future, just as the apostle John does in Revelation 4-22,and the prophet Nephi 1 in 2 Nephi 25-30.
Second, the Book of Mormon prophet Lehi and his family left Jerusalem about 600 B.C. and took with them scriptural writings on plates of brass that contained much of the Old Testament, including Isaiah (1 Ne. 5:13;19:22-23). Book of Mormon prophets taught from the brass plate records, not only from chapters 1-39, which are usually assigned by scholars to the prophet Isaiah of the eighth century B.C., but also from the later chapters, the so-called Deutero-Isaiah. For example, Isaiah chapters 48-54 are all quoted in the Book of Mormon, with some passages mentioned a number of times (1 Ne. 20-21; 2 Ne. 6:16-8:25; Mosiah 12:21-24;14;15:29-31; 3 Ne. 16:18-20;20:32-45;22). Hence, the existence of a virtually complete Isaiah text in the late seventh century B.C., as witnessed by the Book of Mormon, negates arguments for later multiple authorship, whether those arguments be historical, theological, or literary.
Finally, other significant witnesses exist for the single authorship of Isaiah, including Jesus Christ in particular (cf. Matt. 13:14-15;15:7-9; Luke 4:17-19; 3 Ne. 16, 20-22). Indeed, after quoting much from Isaiah 52 (3 Ne. 16:18-20;20:32-45) and repeating Isaiah 54 in its entirety (3 Ne. 22), the resurrected Jesus Christ admonished his Book of Mormon disciples to study Isaiah's words and then said, "A commandment I give unto you that ye search these things diligently; for great are the words of Isaiah. For surely he spake as touching all things concerning my people which are of the house of Israel" (3 Ne. 23:1-2).
Jewish and Christian traditions from the earliest times have supported the single authorship of Isaiah. The Septuagint, the Dead Sea Scrolls, and other ancient texts also give no hint of multiple authorship. Latter-day Saints accept the words of the risen Jesus that Isaiah was a seer and revelator whose prophecies, as recorded throughout his book, will eventually all be fulfilled (3 Ne. 23:1-3). Particularly from Jesus' attribution of Isaiah 52 and 5 4 to the ancient prophet have Latter-day Saints concluded that the book of Isaiah is the inspired work of the eighth-century prophet Isaiah, son of Amoz.