What does the Bible say about taking away from the Bible or adding to the Bible?
You're referring to Revelations 22: 18–19, which evangelicals like to used against Latterday Saints and their Book of Mormon, by extrapolating the word "book" to envelope the whole New Testament or the whole Bible, when by simple logic it appears to limit itself to just the writings of John in the "book" or "scroll" of Revelations itself.
In the KJV, this passage reads.
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.
But in the NIV, it more clearly reads,
I warn everyone who hears the words of the prophecy of this scroll: If anyone adds anything to them, God will add to that person the plagues described in this scroll.
And if anyone takes words away from this scroll of prophecy, God will take away from that person any share in the tree of life and in the Holy City, which are described in this scroll.
Since the overall compendium known as the Vulgate Bible was set until about 400 AD and the Protestant Bible wouldn't be agreed upon by council for another 1200 years after that, it's hard to see how "John" could have been referring to a compendium that didn't exist yet when he wrote those words on his scroll.
Chronologically, we also have the ticklish problem that while Revelations is the last book we encounter in the New Testament, it probably wasn't the last book written. Its date of authorship is thought to have been about 95 AD. Several other NT books are thought to have been written after this. "John's" own Gospel dates from about 90 to 110 AD, Acts from 95 to 110 AD, 1 & 2 Timothy & Titus from about 100 AD, the Letters of John are fuzzy but most scholars put them about a decade after the Gospel so 90 to 110 AD, and 2 Peter is thought to be the youngest "book" of the NT dating from about 110 AD. If they were written after Revelations are we "adding" to the immutable "book" John is talking about? If the King James Bible drops all the extra Old Testament books that the Vulgate includes, is that "taking away" from John's "book"?
I think not in both cases.