Breaking down the outline of Isaiah today in a class. You may have seen divisions of Isaiah into chapters 1-39, 40-55, 56-66. As with most outlines, this captures a truth. Chapters 1-39 are mostly about Isaiah's own time. Chapters 40-55 seem to picture a point right at the end of the exile when Cyrus, king of Persia, allows significant numbers of Jews to return to Jerusalem from Babylon. Then chapters 56-66 seem to picture a time after the return has taken place.
I would rather just divide Isaiah into two parts: 1) chapters 1-39 which relate mostly to the pre-exilic time of Isaiah and chapters 40-66 which relate to the time at the end and after the exile.
Here is a more detailed outline of the content:
Part I: Chapters 1-39
A. Prophecies of Isaiah
1. Isaiah 1-12
This is a concentrated collection of Isaiah's prophesies, including both condemnation for the sins of Israel in Isaiah's day and hope for a king who will restore righteousness. The fear of Assyria is in the background.
2. Isaiah 13-23
Next we have a series of prophecies against various nations, including Babylon, Moab, Damascus, Cush, Egypt, Tyre, and even Jerusalem itself.
3. Isaiah 24-35
These chapters may actually belong with 13-23 but I thought it was worth teasing them out because they deal more broadly with the situation of the world, God's judgment of all the nations, the faithlessness of the leadership of Israel and Judah, and the future restoration of God's people. There are some passages here that were ripe for later eschatological imagery (e.g., chap. 26).
B. Historical Interlude
4. Isaiah 36-39
These chapters are virtually word for word the same as 2 Kings 18-20. Using evidentiary reasoning, we would likely conclude that they were taken from 2 Kings. However, for various reasons, some would strongly want to argue that 2 Kings copied his chapters from Isaiah. These chapters deal with the latter reign of Hezekiah and the surrounding of Jerusalem by the Assyrian king Sennacherib.
Part II: Chapters 40-66
A. Preparing to Return
5. Isaiah 40-55
Suddenly, the context of Isaiah shoots forward two hundred years to the time when Cyrus, king of Persia, allows the Jews to return to Jerusalem from Babylon. Cyrus himself is mentioned in 45:1. Whatever one thinks about authorship of this section (Isaiah is not said to be speaking in these chapters), to understand them we must put ourselves around the year 539BC to read them in context.
B. Hope in Devastation
6. Isaiah 56-66
From a literary standpoint, I don't see as clear a break here inductively, but I haven't studied it thoroughly either. It does seem to me that the tone changes in the last chapters of Isaiah. Whereas 40-55 are hopeful and optimistic, these last chapters have a certain sadness to them at points. It's almost as if the exiles arrived in Jerusalem from Babylon, and found a land laid waste (e.g., 64:10; 61:4; 62:12).