What is significant in there being 66 documents (Books and Letters) in the Holy Bible?

That's the Protestant Bible. The original and true Christian Bible has 73 books. The only "significance" of the 66 books is the fact that Martin Luther trashed seven books of God's Holy Word that every Christian on Earth had used for the previous 1,200 years. He fully intended to trash three New Testament books as well, but was prevented from doing so by his own followers, who were outraged at the idea of trashing the writings of the apostles themselves. If he had carried through his plan, you would be asking what the significance is of the 63 books in the Protestant Bible.