Supposedly the Pistas Sophia (Book of Sophia?) was one of the missing books. However, I'm getting this from a very biased source, so I'm not sure how legitimate it is, but I will quote.
"It was the theologians who created the Greek canons of the Fifth Council, further expounding the Doctrine of Rebirth as a teaching of Jesus Christ, but also as a universal law governing the lives of all humankind:
The savior answered and said unto his diciples: 'Preach ye unto the
whole world, saying unto men, 'Strive together that ye may recieve
the mysteries of light in this time of stress and enter into the kingdom
of light. Put not off from day to day, and from cycle to cycle, in the
belief that ye will succeed in obtaining the mysteries when ye return
to the world in another circle."
I have a theory regarding this "missing book".
(BEWARE: They might go against mainstream Christian thought, so if you are sensitive to other opinions, don't read on!)
HERE IT IS: Jesus really did preach about reincarnation, but the early Church knew that the only way to control their new followers was through fear. The easiest way to generate fear was to make people fear the afterlife so much that they were required to become godly people, supporting the Church and all it stood for, or go to Hell. However, allowing the notion of reincarnation to exist would undermine this fear. Why fear going to Hell for your sins when you believe you will have a second chance to be better in the next life, and will only go to Heaven when you have learned the spiritual truths God intends for you to learn and EARNED your right to be with Him? So, the Church decided that when Jesus said before his death that he would return to the earth one day, he REALLY meant that he would return in the body he was in before he died, which he could do because it would be a miracle of God made possible because Jesus was God's son (or God, depending on your view). And that he would NOT be reincarnated and return that way. Then, to further squash any belief in reincarnation, they neglected to add the Pistas Sophia into the Bible and banned the notion of reincarnation at the Council of Constantinople in 553 C.E. with the words:
"Whosoever shall support the mythical doctrine of the pre-existence of the Soul, and the consequent wonderful opinion of its return, let him be anathema."
(This last really was said... it does not come from some book nobody has ever heard of but from the actual Council of Constantinople.)