"I testify to everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues which are written in this book; and if anyone takes away from the words of the book of this prophecy, God will take away his part from the tree of life and from the holy city, which are written in this book." Revelation 22:18-19
This passage is frequently ripped from its context and cited with regard to the canon of Scripture. People will say things like, "Notice the words, 'the book of this prophecy'? It's equating the canon and prophecy. If there are true prophets today, then the canon of Scripture isn't complete." Those statements are completely false. They assume egregiously and erroneously that "the book" is referring to the entire Bible.
John recorded the Revelation. He then sent it to the churches. It was his letter all by itself. While by application the notion of this passage can certainly be applied to the whole of Scripture—Do not add to it, Do not remove from it; the correct contextual interpretation thereof is that it is speaking concerning the entire prophecy of the book of Revelation. The Greek words translated "this" in both verses are demonstrative pronouns. One is feminine, referring to the prophecy, and the other is masculine, referring to the book. When John sent this letter, the other 65 books were not with it. "This" refers to his letter and its contents, the book of Revelation. Not to the collective Scriptures. This passage is written in regard to the entire book of Revelation, not to the entire canon of Scripture.