Hope it is ok for another non-expert to chime in. There are several "tests" applied to books and among them are:
1) Acceptance by the original audience
2) Dynamic, it changes lives
I was taught 2 or 3 more, but my memory fails.
No, not #2. "Changes lives" is too subjective and is an invitation for loons to think their opinions are authoritative. The books of the New Testament were written by the Apostles, or are approved by the church under the Apostles: Matthew, Mark, Luke, John, Peter, James, Jude, and Paul. Even though there is some dispute, all these men were Apostles, except Mark and Luke. Mark and Luke were close companions to the Apostles.
Books left out of the Bible don't pass the above qualification.