It amazes me the amount of scripture that we know that is missing. There are missing scriptures mentioned in the Bible and in the Book of Mormon. And then there is the sealed portion of the Book of Mormon as well.
Here is what Elder James E. Talmage had to say on the topic:
“Missing Scripture.—Matthew’s commentary on the abode of Joseph, Mary and Jesus at Nazareth, “and he came and dwelt in a city called Nazareth: that it might be fulfilled which was spoken by the prophets, he shall be called a Nazarene” (2:23), with the fact that no such saying of the prophets is found in any of the books contained in the Bible, suggests the certainty of lost scripture. Those who oppose the doctrine of continual revelation between God and His Church, on the ground that the Bible is complete as a collection of sacred scriptures, and that alleged revelation not found therein must therefore be spurious, may profitably take note of the many books not included in the Bible, yet mentioned therein, generally in such a way as to leave no doubt that they were once regarded as authentic. Among these extra-Biblical scriptures, the following may be named; some of them are in existence to-day, and are classed with the Apocrypha; but the greater number are unknown. We read of the Book of the Covenant (Exo. 24:7); Book of the Wars of the Lord (Numb. 21:14); Book of Jasher (Josh. 10:13); Book of the Statutes (1 Sam. 10:25); Book of Enoch (Jude 14); Book of the Acts of Solomon (1 Kings 11:41); Book of Nathan the Prophet, and that of Gad the Seer (1 Chron. 29:29); Book of Ahijah the Shilonite, and visions of Iddo the Seer (2 Chron. 9:29); Book of Shemaiah (2 Chron. 12:15); Story of the Prophet Iddo (2 Chron. 13:22); Book of Jehu (2 Chron. 20:34); the Acts of Uzziah, by Isaiah, the son of Amoz (2 Chron. 26:22); Sayings of the Seers (2 Chron. 33:19); a missing epistle of Paul to the Corinthians (1 Cor. 5:9); a missing epistle to the Ephesians (Eph. 3:3); missing epistle to the Colossians, written from Laodicea (Col. 4:16); a missing epistle of Jude (Jude 3).”Jesus the Christ, James E Talmage, p. 113
And in 2 Nephi 4:1–2, we read:
1 And now, I, Nephi, speak concerning the prophecies of which my father hath spoken, concerning Joseph, who was carried into Egypt.
2 For behold, he truly prophesied concerning all his seed. And the prophecies which he wrote, there are not many greater. And he prophesied concerning us, and our future generations; and they are written upon the plates of brass.
And of course the writings of the isles of the sea (2 Nephi 29:11).
And then the sealed portion of the plates from Ether 4:4–6, we read:
4 Behold, I have written upon these plates the very things which the brother of Jared saw; and there never were greater things made manifest than those which were made manifest unto the brother of Jared.
5 Wherefore the Lord hath commanded me to write them; and I have written them. And he commanded me that I should seal them up; and he also hath commanded that I should seal up the interpretation thereof; wherefore I have sealed up the interpreters, according to the commandment of the Lord.
6 For the Lord said unto me: They shall not go forth unto the Gentiles until the day that they shall repent of their iniquity, and become clean before the Lord.
7 And in that day that they shall exercise faith in me, saith the Lord, even as the brother of Jared did, that they may become sanctified in me, then will I manifest unto them the things which the brother of Jared saw, even to the unfolding unto them all my revelations, saith Jesus Christ, the Son of God, the Father of the heavens and of the earth, and all things that in them are.
My personal belief is that we will someday receive much of these scriptures either again, or for the first time. What a wonderful thing to look forward to!