I suppose this is the verse you are referring to:
Now therefore hearken, O Israel, unto the statutes and unto the judgments, which I teach you, for to do them, that ye may live, and go in and possess the land which the LORD God of your fathers giveth you.
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 4:1-2)
Well, after the Torah several other books were added. This prohibition is once more given in the Book of Revelation:
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.
Books of the Bible have been added after the Torah. They were canonized by Jewish leaders before Jesus came along. The Septuagint version of the old testament was verified by Jewish Leaders and also by Jesus and the Early Church. I am not aware of any accusations that these books added or took away from God's word. We have to keep in mind that the books of the Bible were not given all at one time. God has communicated to mankind by way of revelations over time.
When the New Testament was written it was also given to us over time (from approximately 50 AD to 100 AD). The Holy Ghost has led Christian leaders to have descerment as to which books are inspired and which ones were not. The first Church counsels finally canonized the New Testament as we know it.
The prohibition to add or subtract refers both to books of the Bible and to making changes to the books themselves that would altar the meaning of what God is saying to us.
Look at it this way. It is wrong to add the Book of Mormon as inspired. It would be wrong to remove the Book of Revelation or James from our Bible (although Luther wanted to do that). It would be wrong to add or subtract, or change the meaning of the Sermon on the Mount.
I feel like I'm rambling. Hope this helps.
Ken 