By 'book', he is referring to the Revelation - not the Bible that Jesus taught from.
So it is alright then if people add to and take away from the rest of words God spoke to us???
It was never ok:
Ye shall not add unto the word which I command you, neither shall ye diminish [ought] from it, that ye may keep the commandments of the LORD your God which I command you. – Deuteronomy 4:2