In 2 Nephi 27:7-11, the book that the Lord God "shall bring forth unto you" is spoken of as being "sealed." Because a portion of the gold plates obtained by Joseph Smith was sealed, we have generally identified that portion as the sealed book spoken of in Isaiah 29 and 2 Nephi 27. However, neither Isaiah nor Nephi clearly differentiate between the sealed and unsealed portions of the book. It may be that the "seal" Nephi was speaking of is that the message of the Book of Mormon (even the unsealed and published part) is "sealed" to anyone without faith, to anyone who trusts in the learning of the world and rejects the revelations of God.
Although the book itself (meaning the gold plates) would be "hid from the world," the message of the book (except the sealed portion) is to go forth to all the world. [Robert J. Matthews, "Two Ways in the World," in Studies in Scripture: Book of Mormon, Part 1, pp. 154-155] [See the commentary on Ether 3:25]