Three authors: Proto-Isaiah, Deutero-Isaiah, Trito-Isaiah.
Presumably Isaiah (“proto-Isaiah”) wrote chapters 1–39, an anonymous author living during the exile (“deutero-Isaiah”) wrote 40–55, and another anonymous author living after the exile (“trito-Isaiah”) wrote 56–66.
Scholars generally accept that the book of Isaiah was at least partially written by the prophet Isaiah, during the eighth century BC. However, there’s also evidence that other authors made additions.
There are numerous challenges in Isaiah that lead scholars to speculate about which parts were written by Isaiah himself, and what else was added and by whom.
Most scholars agree that the prophet Isaiah likely only wrote a portion of the book, but recent scholarship also argues that even the portions he didn’t physically write originated with him in some form.
https://zondervanacademic.com/blog/who-wrote-isaiah
The book of Isaiah was written by the prophet Isaiah. The book is categorized as one of the Major Prophets in the Old Testament or, in the Hebrew Tanakh, as a Latter Prophet.
The term major refers to the length of the books rather than the credentials or importance of the authors. In the Tanakh, prophets like Isaiah, Jeremiah, and Ezekiel are termed “Latter Prophets” to distinguish them from the Former Prophets such as Samuel and Nathan.
Isaiah prophesied during the time of the divided kingdom under the reigns of Uzziah, Jotham, Ahaz, and Hezekiah, kings of Judah. Isaiah likely prophesied between 739—681 BC, a time when both the northern and southern kingdoms had turned away from God.
Israel fell to Assyria in 722 BC, and Judah continued its trajectory of rebellion against God. Tragically, the warnings of Isaiah and other prophets changed nothing, and Judah later fell to the hands of Babylon.
Messianic prophecies
Isaiah also contains many Messianic prophecies, including Isaiah 7:14; 9:6–7; and 11:1–5. The prophet probably died during the reign of Manasseh. Tradition states that Isaiah was martyred.
https://www.gotquestions.org/who-wrote-the-book-of-Isaiah.html
investigative Tomas Fredriksson 