The Bible says in several places not to add to his words,

how can this be true when other books were added afterwards?

What it means is that one is not to add their own words to what God gave. Each one is a specific book that was complete that man was not to add or subtract from. The Bible is a collection of all 66 books that are of the Old and New Testament.

Deuteronomy 4:2 “You shall not add to the word which I command you, nor take anything from it”(also Deuteronomy 12:32). The reason God is so adamant on this is because “The entirety of Your word is truth” (Psalms 119: 160).

"Add thou not unto His words, lest He reprove thee, and thou be found a liar" Proverbs 30:6.

  In our day there is the belief in “progressive revelation” that originates from outside the Word. What right does anyone have to teach what is not in Scripture as if it is Scripture?  False teachers do not want to submit to its original intent, so they do not seek what Scripture actually (exegesis) means, instead, they conform it to what they want it to mean (isogesis).  They will look to another source as their authority because they are not under God’s authority. This is why Paul admonishes us in 1 Corinthians 4:6: “not to think beyond what is written” (exceed or go past). Why? So pride will not have an opportunity to operate and puff us up. It is pride that makes one participate in false doctrine which makes him spiritually destructive to himself and others who listen to him.

The reason God is so adamant on this is because “The entirety of Your word is truth” (Psalms 119: 160). Proverbs 30:5-6: “Every word of God is pure; he is a shield to those who put their trust in Him.”

If we look at the context it saying “Every word of God.” God is stating for MAN not to add to his words. Adding does not only mean additional words, but can also mean changing them to mean what they do not. When you add new words as equal to Scripture you are really taking away from Scripture.

In the book of Revelation which is prophecy completes the Bible, just as Genesis began the Bible, it states "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" Revelation 22:18-19.

Of all the books Revelation has the strongest warning as it has to do with the end of world system and the coming of the Lord to restore righteousness