First, Second, and Third Isaiah are all one fella
Reading today about the unity of the book of Isaiah:
• 1-35 primarily about the immediate crisis of the Assyrian invasion but here and there references to Babylon and the Messiah, 36-39 historical transition from immediate crisis to later problem of the exile, 40-66 the return from exile and the work of the Messiah. 36-39 is the hinge of the two halves of the book.
• Prophet Jeremiah apparently alludes to Isaiah 40-66 again and again.
• Plus, our earliest copy of Isaiah (ca. 125 BCE), the great scroll of Isaiah of the Dead Sea Scroll (1QIsa), ends chap. 39 on the next-to-last line of a column. Chap. 40 starts on the next line with no special markings, space, etc.
The MS it was copied from also had the entire text, which takes us back another 50+ years.
• Characteristic phrasings occur throughout 66 chapters.
• NT quotes from all sections as from Isaiah.
Want to go deeper?
For more, see EJ Young, Isaiah [http://ow.ly/ydQfk], 3:538-549; EJY, Who Wrote Isaiah?; R. Margalioth, Indivisible Isaiah. I’m convinced.