Where in the Bible does it say that there is a Book of Mormon in addition to the Bible?
The references to the Book of Mormon in the Bible are rather obscure and need a little interpretation. Here is a specific reference from Ezekiel 37:15-20.
The word of the LORD came again unto me, saying,
Moreover, thou son of man, take thee one stick, and write upon it, For Judah, and for the children of Israel his companions: then take another stick, and write upon it, For Joseph, the stick of Ephraim, and for all the house of Israel his companions:
And join them one to another into one stick; and they shall become one in thine hand.
And when the children of thy people shall speak unto thee, saying, Wilt thou not shew us what thou meanest by these?
Say unto them, Thus saith the Lord GOD; Behold, I will take the stick of Joseph, which is in the hand of Ephraim, and the tribes of Israel his fellows, and will put them with him, even with the stick of Judah, and make them one stick, and they shall be one in mine hand.
And the sticks whereon thou writest shall be in thine hand before their eyes.
The word “stick”: In the time of the writing of this scripture, and carried on even today in some of the Jewish synagogues, the scriptures were written on scrolls wound on a rod or “stick.” So the phrase about writing upon a stick actually means writing upon a parchment or papyrus that was scrolled onto a stick. This scripture talks about two sticks, or scrolls, or if you will, books. The stick written on for Judah, also known as the stick of Judah, is the Bible, as it is a record of the descendants of Judah, or the Jews, and their companions among the children of Israel. The tribes of Judah and Benjamin separated themselves from the other ten tribes of Israel and settled in Jerusalem, and the Books of the Bible are mostly associated with this branch of the House of Israel.
The other stick which was written on for Joseph is called the stick of Ephraim, and concerns the part of the ten tribes who remained in a group following the exodus of the tribes of Judah and Benjamin. The two sons of Joseph were Ephraim and Manasseh.
In 600 B.C., a group of people, who were descendants of Ephraim and Manasseh, left Jerusalem under the guidance and direction of the Lord and were led to the New World–that which is now North and South America. God continued to reveal himself to these people and instruct them through their prophets. They kept a record of God’s dealings with them for the period of 1000 years. This record was kept on metal plates and was inscribed using Egyptian hieroglyphs. These records were abridged by the second-to-last prophet of these people, whose name was Mormon, and were hid up in a hill situated near modern-day Palmyra, New York. In the year 1823, an angel of the Lord appeared to a young man by the name of Joseph Smith and later placed these records in his hands. Joseph Smith, under the inspiration of God, translated them into the English language, and this abridged record of the ancient inhabitants of the American continent, who were descendants of Ephraim and Manasseh, was called The Book of Mormon.
Ezekiel speaks of putting the two books together and make them one stick, and they shall be one in mine hand. And the sticks whereon thou writest shall be in thine hand before their eyes. This reference explains that the Bible and the Book of Mormon are as one book in the hands of the Lord. In other words, they both preach the same gospel, they both testify of the divinity and of the great atoning sacrifice of the Lord, Jesus Christ, and they both cite the same priesthood authority given by God to his prophets on both hemispheres. The Book of Mormon is known as a second witness for Jesus Christ, and the Bible and Book of Mormon are carried and used together by the missionaries and members of the Church of Jesus Christ of Latter-day Saints, thus fulfilling the scripture And the sticks whereon thou writest shall be in thine hand before their eyes.