What is the significance of the number 66?
39 + 27? = 66 chapters in Isaiah = 66 books in the Bible
“To understand the book of Isaiah, we need to have the view from the entire sixty-six books of the Bible. Isaiah is of two portions.
The first portion includes chapters one through thirty-nine, and the second portion includes chapters forty through sixty-six. The first thirty-nine chapters of Isaiah correspond to the thirty-nine books of the Old Testament. The last twenty-seven chapters correspond to the twenty-seven books of the New Testament. The New Testament begins with John the Baptist (Mark 1:1-3), and Isaiah 40 also begins with John the Baptist (v. 3).”
(Life-Study of Isaiah, Chapter 18)