Regarding the Book of Mormon: It is different. God preserved it for a purpose in Himself. This book, called the Book of Mormon, is an abridgment prepared by one of the last prophets of the Nephites, under the command of God, that it might come forth in the last days. God revealed in part to him, and to his son Moroni, the purpose which He had in view, in making this abridgment, and in concealing it in the earth, and they performed the labor connected with this under the direct command and inspiration of the Almighty, to come forth in the latter times, and to accomplish a great work.
And now I, Mormon, being about to deliver up the record which I have been making into the hands of my son Moroni, behold I have witnessed almost all the destruction of my people, the Nephites.
And it is many hundred years after the coming of Christ that I deliver these records into the hands of my son; and it supposeth me that he will witness the entire destruction of my people. But may God grant that he may survive them, that he may write somewhat concerning them, and somewhat concerning Christ, that perhaps some day it may profit them.
And now, I speak somewhat concerning that which I have written; for after I had made an abridgment from the plates of Nephi, down to the reign of this king Benjamin, of whom Amaleki spake, I searched among the records which had been delivered into my hands, and I found these plates, which contained this small account of the prophets, from Jacob down to the reign of this king Benjamin, and also many of the words of Nephi.
And the things which are upon these plates pleasing me, because of the prophecies of the coming of Christ; and my fathers knowing that many of them have been fulfilled; yea, and I also know that as many things as have been prophesied concerning us down to this day have been fulfilled, and as many as go beyond this day must surely come to pass—
Wherefore, I chose these things, to finish my record upon them, which remainder of my record I shall take from the plates of Nephi; and I cannot write the hundredth part of the things of my people.
But behold, I shall take these plates, which contain these prophesyings and revelations, and put them with the remainder of my record, for they are choice unto me; and I know they will be choice unto my brethren.
And I do this for a wise purpose; for thus it whispereth me, according to the workings of the Spirit of the Lord which is in me. And now, I do not know all things; but the Lord knoweth all things which are to come; wherefore, he worketh in me to do according to his will.
And my prayer to God is concerning my brethren, that they may once again come to the knowledge of God, yea, the redemption of Christ; that they may once again be a delightsome people.
And now I, Mormon, proceed to finish out my record, which I take from the plates of Nephi; and I make it according to the knowledge and the understanding which God has given me.
These were the promises of God to the ancient Prophets of this continent.
Source: Words of Mormon 1-9; The Book of Mormon: Another Testament of Jesus Christ