Why the 'Holy Bible' has 66 books. No more, no less.

Each book must pass the test for canonicity. See "Encountering the Old Testament" 2nd ed. x Arnold & Beyer
  1. Test #1: Written by a prophet or Spirit-led person
    Human authors would not know God's will apart from God's Spirit assisting their understanding. The Spirit of God had to guide the writing process. His presence ensured the finished product was God's truth and accurately communicated God's message.
  2. Test #2: Written to all generations
    God's message could not be confined to one audience. If a book was God's Word, it must be relevant to all people of all times. The author might have written the work for a particular audience, but if it was God's word, all who read it could profitably apply its teaching to their lives.
  3. Test #3: Written in accord with previous revelation
    If a new writing claimed to be from God but contradicted the teachings of Genesis, it could not be God's Word. God's truth remained the same and would not contradict itself. New revelation could reveal further information about God's plan and purposes, but it would never run counter to the old.