If the Bible is incomplete, then we can only get a partial view of the Word of God(Jesus-John 1:14, 5:39, Heb 10:7, etc). How can we be changed into a complete image, beholding the glory of the Lord, if the image is missing pieces? The Bible has a beautiful truth in the following Scripture, so we can be sure God included all the books that were supposed to be included in the Bible:
"But we all, with open face beholding as in a glass the glory of the Lord, are changed into the same image from glory to glory, even as by the Spirit of the Lord." 2 Cor 3:18