So let’s get this straight. There are three, not one, prophet collections named Isaiah, and all are recorded in the single Old Testament book of the same name.
First Isaiah, or Isaiah of Jerusalem, was a contemporary of Amos, Hosea and Micah. His career spanned four decades, during which he founded a school of prophecy. “The record is to be folded and the sealed instruction kept among my disciples.” (Isaiah 8:16). His words were valued as living texts, updated and re-interpreted in the wake of later events, chiefly the Babylonian Exile. Scholars today believe less than half of the actual words attributed to First Isaiah were his; the rest were added by anonymous scribes.
Second Isaiah essentially was written after the exile to Babylon and predicts the restoration of Jerusalem. Third Isaiah dates from the period after exile ends. They inherited the spirit of Isaiah of Jerusalem and continued his work.
Here’s the outline derived from critical scholarship:
Chapters 1-39: First Isaiah/Isaiah of Jerusalem, 8th century BC.
Chapters 40-55: Second Isaiah/Isaiah of Babylon, 6th century BC.
Chapters 56-66: Third Isaiah/Isaiah of the restored Jerusalem, 6th century BC.
The critical approach to the Bible, says John J. Collins (“Isaiah,” The Collegeville Bible Commentary, 411), enriches our understanding of Scriptures. It shows how the word of God is rooted in and speaks to concrete historical situations. “The Bible is not a book of dogmatic propositions to be learned and believed, but a moving illustration of the faith of a people in ever-changing circumstances.”