It's not the book itself but the teaching it contains. The first Christians had Jesus' and the apostles' teaching from the very beginning.
Ephesians 2:19-22 says, "So then, you are no longer strangers and aliens, but fellow citizens with the saints and members of the household of God, being built upon the foundation of the apostles and prophets, Jesus Christ Himself being the chief cornerstone, in whom the whole building, being fitted together, is growing into a holy temple in the Lord, in whom you also are being built together into a dwelling place of God in the Spirit."
So we see the church would be nothing without the teachings of Christ, the apostles and the prophets.
I see where you're coming from though. You mean the Bible is the word of God and the truth, but that it's insufficient. Teachers of "new revelation" always say their teaching is in 100% agreement with the scriptures but I haven't found this to be the case. They always twist the scriptures and wander off into error of one kind or another and call it a word from the "Spirit."
That's not a solid foundation to stand on. Go ahead and stand on if you want, but I'll pass. There are already too many false teachers out there.