Rev. 22:18-19
18For I testify to everyone who hears the words of the prophecy of this book: If anyone adds to these things, God will add to him the plagues that are written in this book; 19and if anyone takes away from the words of the book of this prophecy, God shall take away his part from the Book of Life, from the holy city, and from the things which are written in this book.
What "book" was John referring to?
I am reminded of a passage from the book of Revelations, (a book of the Bible which was intentionally left out of this translation, while including the gnostic gospel of Thomas):
For you say, I am rich, I have prospered, and I need nothing; not knowing that you are wretched, pitiable, poor, blind, and naked. Therefore I counsel you to buy from me gold refined by fire, that you may be rich, and white garments to clothe you and to keep the shame of your nakedness from being seen, and salve to anoint your eyes, that you may see.
Revelation 3:17, 18