In the Book of Mormon, there is found a revelation from God in which He proclaims that when the verses in the Holy Bible were first delivered to ancient prophets, they were perfect:
…and at the time they proceeded out of the mouth of the Jew, or, at the time the book proceeded out of the mouth of the Jew, the things which were written were plain and pure, and most precious and easy to the understanding of all men (1 Nephi 14:23).
Over the thousands of years since the prophets recorded the voice of God, some scriptures have been lost, some mistranslated, and some purposefully changed.
Margaret Barker, a Baptist and renowned biblical scholar, has given lectures regarding the scriptures found by the ancient king of Judea, Josiah. The temple had fallen into such disrepair that Josiah, a righteous man, ordered it cleaned and renovated. The priests of the temple brought the scriptures found within the edifice to Josiah, who rent his clothes when he read the book of the law (of Moses) and realized how far his people had strayed from the commandments and holy ordinances of God.
Dr. Barker, in her research, has been able to surmise that the scriptures given to Josiah had been altered, and the verses testifying of revelation and the ministering of angels removed. These differences in various copies of the scriptures fed future controversies before the Babylonian captivity and between the Pharisees and Sadducees.
Barker sees in the Book of Mormon, a return to the pure scripture of Old Testament times. The prophet Lehi, who escaped from Jerusalem before the Babylonians destroyed the city, brought scriptures with him to the Americas. They seemed to be a Josephite record (and thus specific to the northern kingdom of Israel, already destroyed by the Assyrians), and they contained a genealogy of Lehi’s forefathers, descended from Joseph. The Book of Mormon peoples and their prophets, living according to the Law of Moses and looking forward to the coming of Jesus Christ, used these scriptures and always had as part of their worship and gospel, revelation to prophets and the ministering of angels.
Some books of scripture are mentioned in the Bible in such a way that it seems they were considered authentic and valuable, but they’re not found in today’s Bible.  Note that most of these are Old Testament books. Sometimes called missing scripture, the following books are some of the ones that have been lost and may someday be recovered:
- the Wars of the Lord (Numbers 21:14)
- the Book of Jasher (Joshua 10:13; Samuel 1:18)
- the Acts of Solomon (1 Kings 11:41)
- the Book of Samuel the Seer (1 Chronicles 29:29)
- the Book of Gad the Seer (1 Chronicles 29:29)
- the Book of Nathan the Prophet (1 Chronicles 29:29; 2 Chronicles 9:29)
- the Prophecy of Ahijah (2 Chronicles 9:29)
- the visions of Iddo the Seer (2 Chronicles 9:29)
- the Book of Shemaiah (2 Chronicles 12:15)
- the Book of Jehu (2 Chronicles 20:34)
- Sayings of the Seers (2 Chronicles 33:19)
- an Epistle of Paul to the Corinthians predating 1 Corinthians (1 Corinthians 5:9)
- possibly an earlier epistle to the Ephesians (Ephesians 3:3)
- an epistle to the Church at Laodicea (Colossians 4:16)
- prophesies of Enoch known to Jude (Jude 1:14)
Also, four hundred years passed from the record of Malachi the prophet in the Old Testament and the birth of our Lord and Savior Jesus Christ. During that time a series of wicked kings reigned in Judea, especially Manasseh, who ruled for fifty years and killed every prophet he could find. One would suppose that because there were prophets during these 400 years, that they wrote the revelations they received from God, and that if we do not have them, they are either hidden and still to be discovered, or they have been destroyed.
This attests to the fact that our present Bible does not contain all of the word of the Lord that He gave to people in former times, and they remind us that our Bible is incomplete. Matthew mentioned a prophecy in Jewish scripture that predicted Christ would be a Nazarene. We do not have that scripture.
The Book of Mormon makes reference to scripture of Old Testament times that are not found in the Bible or in the Book of Mormon. References are made to the unknown writings of Zenock, Zenos, and Neum, extensive writings and prophecies of Joseph who was sold into Egypt, and a prophecy of Jacob. These writings were probably found in the scriptures had among the Book of Mormon peoples.
The Book of Mormon is an abridgment of important scriptures, and much has been held back by the Lord for a later time, when people are worthy to receive them. Part of the Book of Mormon has been sealed, and Joseph Smith was not allowed to translate it. Included in that portion are the writings of the Brother of Jared (Mahonri Moriancumr), who saw “the beginning from the end.” The Book of Mormon also mentions that the Ten Lost Tribes who were led away by the Lord have their own scriptures, which they will bring with them when they miraculously return (2 Nephi 29).
The Doctrine and Covenants mentions the writings of John the Baptist, and a full account of the experience at the Mount of Transfiguration, which scripture has not yet been entrusted to men.
See more at the LDS Bible Dictionary.