Here is an interesting take on why 66 books in non Roman bible. The Tanakh having 39. The new testanent canon having 27. Whether by pure coincidence or not, The Book of Isaiah has 66 chapters. The first 39 are analogous to the 39 of the Tanakh. Chapter 40 of Isaiah is Messianic. Containing the familiar passage “The voice of him that crieth in the wilderness, Prepare ye the way of the Lord, make straight in the desert a highway for our God.”
The gospel of Mark opens with a quote of this Isaiah passafe. Thus chapters 40-66 (27 in total) are analogous to the 27 books of the New Testament canon.
Cool, seems once I learned of fore-shadowing, more and more examples continue to be revealed.