The "book" existed in part BEFORE Jesus was incarnated. The rest was written not that long after He ascended and it contains ALL that Jesus taught of the Christian faith to those actual people. His Apostles and disciples ensured that the truth was written and preserved for all future generations and they were led by the Holy Spirit in exactly what should be written because it is the word of God. He gave us an objective and authoritative resource by which all claims to truth could be measured - up to and including those who later asserted they were the only authority.
It is better to trust in the LORD than to put confidence in man. (Psalm 118:8)
John 21:25 And there are also many other things which Jesus did, which if they were written in detail, I suppose that even the world itself would not contain the books that would be written.
Well that was an easy one...