Adding to or Ignoring God's word

Deuteronomy 12:32 “Whatever I command you, be careful to observe it; you shall not add to it nor take away from it.”

Proverbs 30:5-6 “Every word of God is pure; he is a shield to those who put their trust in Him. Do not add to His words, lest He rebuke you, and you be found a liar.”

But theses are old testament.  So the new testament was added.  So my question isis the book of Mormon valid or invalid.  There are two groups I am aware of that use the book of Mormon.  The Church of Jesus Christ of Later Day Saints, and the Church of Jesus Christ.  I received a book on Joseph Smith from a local LDS church.  After reading a few chapters my feeling was that Joseph Smith was far to self serving.  Next couple in the extreme secrecy of the group and I feel it is not the place for me.  I feel secrecy is primarily a tool for satan.

If someone can justify the book of Mormon I would love to see the referring scripture.  I read Ezekiel 37, but I do not see it.