Isaiah was lived from 740 BC to 700 BC and wrote a Book of Isaiah, but his original one is from chapter 1 to 39. Chapter 40 to 55 was written during the end of Babylon captivity by some others. 56 to 66 was written later by others.

Of course we have been not discussing historical accuracy or scholastic view on Bible, but in fact the tone of a book drastically changes from chapter 40 and not single name Isaiah appears from chapter 40.

Basically prophecy of Hebrew Bible is a presentation of hope and Bible was written over a long period of time