Or rather, which of the writings attributed to him are really his?
Scholars have recognized for centuries that the single work as we have it is the composition of several writers over two or three centuries, maybe more.
First Isaiah, found in Chapters 1-39, contains what most scholar believe to be the true writings of the eighth century prophet himself.
However, even there not everything is the work of Isaiah; to him belong chapters 1-12, 15-23, and 28-33.
Among that first set is an interpolation found in Isaiah 2:2-5, copied from Micah 4:1-5.
The oracles against Babylon in Isaiah 13-14, the “Apocalypse of Isaiah” in Isaiah 24-27.
The poems in Isaiah 34-35 were written by disciples of his or close associates.
The passages in Isaiah 36-39 were adapted entirely from 2 Kings 18-20.
Second Isaiah, Chapters 40-55, was written by an anonymous poet-prophet in the early post-Exilic period. It includes the verse (Isaiah 45:7) that is the basis for the Yotzer ohr prayer benediction before the Shema of shacharit (Jewish morning prayer), the Servant Songs, and several “prophecies” classified as vaticinium ex eventu. That many of these “prophecies” are of Koroush Kabir of Iranshar (Cyrus the Great of Iran) allowing the exiles to begin returning bears witness to their actual date of composition.
Third Isaiah was the work of several different writers over a broad period extending well into the Hellenistic era.