In 2 Nephi 27:13-14, the Lord showed that there are to be three major written testimonies of Christ to come forth: (1) the Bible (of the Jews), (2) the Book of Mormon (of the Nephites), and (3) the records of the lost tribes of Israel, which we do not yet have. These three records are to be shared by each group so that each will eventually have the records of the others. And when Israel is gathered, the records will also be gathered. By these three records, the law of witnesses will be honored and the word of God established.
According to Robert Matthews, these records do not consist only of single books from each major branch of Israel. Just as the Bible is a collection of many books, and the Book of Mormon is a collection also, likewise the records of the ten lost tribes will probably be a collection of various books. It will tell us, when it is received, about the Savior's visit to the ten tribes after his appearance to the Nephites. However, since Ephraim was the leading tribe among the lost tribes anciently, perhaps our present Doctrine and Covenants should be viewed in perspective as a witness for Jesus Christ among latter-day Ephraim, to be combined someday with the other records of the lost tribes when they come forth.
Among the things to be accomplished by the multiple books of scripture are: (1) to give a multiple witness of Christ; (2) to show that the Lord does speak the same gospel to one people as to another, and (3) to prove that God does honor his covenant that he made with Abraham, promising to remember his seed forever (2 Nephi 29:14). [Robert J. Matthews, "Two Ways in the World," in Studies in Scripture: Book of Mormon, Part 1, pp. 158-159]