I believe it is referring to the BOOK, the Bible - all of Scripture. Revelation is the capstone of the Bible, but it is the sum and concluding of all the rest of the Book.

There are warnings in the beginning of the Bible, in the middle, near the end of the OT, and now at the end of the NT.

Deuteronomy 4:2 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 12:32 What thing soever I command you, observe to do it: thou shalt not add thereto, nor diminish from it.

Proverbs 30:5-6 Every word of God is pure: he is a shield unto them that put their trust in him. Add thou not unto his words, lest he reprove thee, and thou be found a liar.

Jeremiah 26:2 Thus saith the LORD; Stand in the court of the LORD'S house, and speak unto all the cities of Judah, which come to worship in the LORD'S house, all the words that I command thee to speak unto them; diminish not a word:

2 Peter 3:16 As also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction.

Revelation 22:18-19 For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book: And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.

1) The word prophecy is used in at least one place in Revelation to refer to ALL of the Word of God:

Revelation 19:10 And I fell at his feet to worship him. And he said unto me, See thou do it not: I am thy fellowservant, and of thy brethren that have the testimony of Jesus: worship God: for the testimony of Jesus is the spirit of prophecy.

The testimony/witness of Jesus is the heart of the whole Bible (my paraphrase).

2) Those other warnings spaced throughout the Bible show a principle of not adding to or taking away from ANY of the Bible - so even if, strictly speaking, it is directly referring to the book of Revelation, the application is to the WHOLE Bible.