During the apostasy, precious truths were lost or removed from the Bible either through design or error. Part of the Restoration included God's revealing of the Book of Mormon. It doesn't take away from but complements the Bible, restoring precious truths and confounds false doctrine.
"We believe the Bible to be the word of God as far as it is translated correctly; we also believe the Book of Mormon to be the word of God," Articles of Faith 1:8
The unveiling of the Book of Mormon should not have been a surprise, because it was prophesied in the Old Testament.
"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." Ezekiel 37:16-17
The Book of Mormon is another testament of Jesus Christ and contains the fulness of His gospel, Doctrine and Covenants 20:8-16. We are here on earth as a test to determine if we will find and follow Jesus, which can be helped along with scripture study including the Book of Mormon.
"For we labor diligently to write, to persuade our children, and also our brethren, to believe in Christ, and to be reconciled to God; for we know that it is by grace that we are saved, after all we can do. And we talk of Christ, we rejoice in Christ, we preach of Christ, we prophesy of Christ, and we write according to our prophecies, that our children may know to what source they may look for a remission of their sins." 2 Nephi 25:23,26